A 38 mm spring band clamp will happily hold a hose that measures 37.0 mm across, and one that measures 41.5 mm. Give it a hose at 36 mm or 43 mm and it either rattles or refuses to go on. That 4.5 mm window is the whole sizing story, and it is why a spring band hose clamp size chart has to read differently from any worm gear chart you have used before.
A spring band clamp has no screw and nothing to adjust. Its size is fixed when it is heat treated: a relaxed (free) inside diameter, plus a small working window above it where the band grips without being overstressed. Miss the window and no effort fixes it. Below: window data from real maker tables, a coolant hose swap sheet, a measuring routine for spring clamps, and four worked examples.
Free diameter vs fitted diameter: the one idea behind spring clamp sizing
Every spring band clamp has three diameters that matter. Makers name them differently:
- Free (closed) diameter: the inside diameter of the clamp sitting on the bench with nothing inside it. This is always smaller than the hose it is meant for. EFC International lists it as “closed diameter”; Rotor Clip calls it D1.
- Minimum functional diameter: the smallest fitted diameter at which the band is stretched far enough to deliver its rated clamping load. Below this, the clamp still closes on the hose, but it presses too lightly.
- Fully opened diameter: the largest diameter the band can be spread to without exceeding its design stress. Above this, you are bending the spring steel past where it was meant to go, and it may take a set or crack at the tabs.
The working window sits between the last two numbers. Your installed hose outside diameter (OD) must land inside it. The nominal diameter stamped on the band (the “38” in a 38 mm clamp) is a label near the bottom of that window, not the middle and not the free size.
Take the 38 mm clamp in EFC International’s standard spring band constant tension clamp listings. Its closed diameter is 34.5 mm, its minimum functional diameter is 37.0 mm and it opens fully to 41.5 mm. So a hose at 37.0 mm stretches the band 2.5 mm past its free size; a hose at 41.5 mm stretches it 7.0 mm. Everything between those two points works.
Würth’s 38 mm DIN 3021 Form A spring band clamp lists the same 37 to 41.5 mm working range, with a 12 mm band, 1.7 mm material and a 300 N minimum spring force. Two catalogs, same window: this is how DIN-style parts are specified.
Key point: too big a clamp means the hose OD sits below the minimum functional diameter and the band barely presses. Too small a clamp means the hose OD sits above the fully opened diameter, so the band is overstressed and very hard to fit. A worm gear clamp forgives both mistakes; a spring band clamp forgives neither.
How wide is a typical spring clamp working window?
From the maker data read for this article, typical windows run like this (labelled typical, check the data sheet for your exact part):
- Small sizes (13 to 20 mm nominal): about 1.5 to 2.5 mm wide.
- Mid sizes (22 to 40 mm nominal): about 2.7 to 4.5 mm wide.
- Larger sizes (42 to 60 mm nominal): about 4 to 5.5 mm wide.
- Some makers publish a tighter “clamping range” than the full window. Rotor Clip’s CTB-29, for example, lists 28.8 to 30.5 mm (1.7 mm), while a DIN-style 29 mm part in the EFC table runs 28.0 to 31.5 mm (3.5 mm).
A worm gear clamp can cover 0.81 to 1.50 in (21 to 38 mm) in one part; a spring clamp covers roughly a tenth of that. Expect more part numbers and tighter measuring.
Spring band hose clamp size chart (mm): nominal size, working window, band thickness
The chart below re-tabulates the published values for one standard 51CrV4 spring steel series sold by EFC International, which states the material as heat-treated alloy spring steel and the service range as -40 to 392 °F. Inch conversions are ours (1 in = 25.4 mm), rounded to two places. The window columns are the minimum functional diameter and fully opened diameter, which is the range your installed hose OD must fall into.
| Nominal (mm) | Free / closed ID (mm) | Working window, hose OD (mm) | Window (in, approx.) | Material thickness (mm) |
|---|---|---|---|---|
| 13 | 12.0 | 12.7 – 14.2 | 0.50 – 0.56 | 0.70 |
| 14 | 13.3 | 13.7 – 15.8 | 0.54 – 0.62 | 0.8 |
| 15 | 13.9 | 14.7 – 16.5 | 0.58 – 0.65 | 0.8 |
| 16 | 14.9 | 15.7 – 17.5 | 0.62 – 0.69 | 1.0 |
| 17 | 15.2 | 16.3 – 18.5 | 0.64 – 0.73 | 0.8 |
| 19 | 17.8 | 18.3 – 20.2 | 0.72 – 0.80 | 1.3 |
| 20 | 18.4 | 19.3 – 21.6 | 0.76 – 0.85 | 1.3 |
| 22 | 20.5 | 21.3 – 24.2 | 0.84 – 0.95 | 1.3 |
| 24 | 22.0 | 23.3 – 26.0 | 0.92 – 1.02 | 1.3 |
| 25 | 23.5 | 24.0 – 26.8 | 0.94 – 1.06 | 1.5 |
| 27 | 25.2 | 26.0 – 29.2 | 1.02 – 1.15 | 1.6 |
| 29 | 27.0 | 28.0 – 31.5 | 1.10 – 1.24 | 1.7 |
| 30 | 28.0 | 29.0 – 32.5 | 1.14 – 1.28 | 1.7 |
| 32 | 29.3 | 31.0 – 34.5 | 1.22 – 1.36 | 1.7 |
| 35 | 31.5 | 34.0 – 38.0 | 1.34 – 1.50 | 1.7 |
| 36 | 32.5 | 35.0 – 39.0 | 1.38 – 1.54 | 1.7 |
| 38 | 34.5 | 37.0 – 41.5 | 1.46 – 1.63 | 1.7 |
| 40 | 35.5 | 39.0 – 42.5 | 1.54 – 1.67 | 1.7 |
| 42 | 37.2 | 40.5 – 44.5 | 1.59 – 1.75 | 1.9 |
| 43 | 37.9 | 41.5 – 45.5 | 1.63 – 1.79 | 1.9 |
| 47 | 41.4 | 45.5 – 50.0 | 1.79 – 1.97 | 2.0 |
| 50 | 43.5 | 48.5 – 53.0 | 1.91 – 2.09 | 2.2 |
| 55 | 43.5 | 53.5 – 58.0 | 2.11 – 2.28 | 2.2 |
| 60 (listed 60.6) | 51.5 | 58.5 – 64.0 | 2.30 – 2.52 | 2.7 |
Würth’s DIN 3021 Form A parts match the window column: its 20 mm part lists 19.3 to 21.6 mm and its 60 mm part 58.5 to 64 mm. Its 23 mm part (22.3 to 24.7 mm) fills a size the table skips.
DIN 3021 spring band clamp band widths and thickness steps
The thickness column follows a stepped pattern that comes from the standard. The DIN 3021-1 Type A dimension sheet (1999 edition, as reproduced by Fasten.it) groups nominal diameters into bands of material thickness and offers two band widths, 12 mm and 15 mm.
| Nominal diameter group (mm) | Band thickness s (mm) | 12 mm band offered | 15 mm band offered |
|---|---|---|---|
| 13 – 18 | 0.8 | Yes | Yes |
| 19 – 25 | 1.3 | Yes | Yes |
| 26 – 40 | 1.7 | Yes | Yes |
| 41 – 55 (12 mm band from 42) | 2.1 | Yes | Yes |
| 58 – 75 (12 mm band from 60; 15 mm band to 67) | 2.6 | Yes | Yes, to 67 |
| 80 – 90 | 2.6 | Yes | No |
Real catalogs stray from this grid. EFC also lists 8.4, 10 and 12 mm wide families, and Rotor Clip describes its CTB family as 15 mm wide, with a 12 mm CTN option (13 to 90 mm nominal) where 15 mm is too wide. Thickness can run a step off the DIN groups too (EFC’s 42 and 43 mm parts are 1.9 mm). When width or thickness matters, read the actual part’s sheet.
One sentence on markings: the number stamped on the band is the nominal diameter, and our guide on how to choose a spring band hose clamp covers reading those stamps, finishes and band width choice in depth.
Coolant hose swap sheet: spring hose clamp sizes by hose ID
Most people looking up a radiator hose spring clamp size know the hose ID, not the OD. This sheet bridges the gap with Gates published ODs, then picks the nominal size from our spring band hose clamp size chart whose window holds that OD with room on both sides.
Gates lists its 1 in ID 4175 standard coolant hose at 1.42 in OD with a 0.195 in wall, and the same wall carries through its 3/4, 1-3/8, 1-3/4 and 2 in sizes. Where we did not read a published OD (1-1/4 and 1-1/2 in), we added two walls of 0.195 in to the ID and marked the result as calculated. The 5/8 in row uses Gates 4230 heater hose at 0.94 in OD, a thinner wall than 4175.
| Hose ID | Published hose OD (free) | Typical installed OD on spigot (approx.) | Suggested nominal spring clamp | Clamp window (mm) |
|---|---|---|---|---|
| 5/8 in (heater) | 0.94 in / 23.9 mm (Gates 4230) | 24 – 25 mm | 24 or 25 | 23.3 – 26.0 / 24.0 – 26.8 |
| 3/4 in | 1.14 in / 29.0 mm (Gates 4175) | 29 – 30 mm | 29 or 30 | 28.0 – 31.5 / 29.0 – 32.5 |
| 1 in | 1.42 in / 36.1 mm (Gates 4175) | 36 – 37 mm | 36 | 35.0 – 39.0 |
| 1-1/4 in | about 1.64 in / 41.7 mm (calculated) | 42 – 43 mm | 42 or 43 | 40.5 – 44.5 / 41.5 – 45.5 |
| 1-3/8 in | 1.80 in / 45.7 mm (Gates 4175) | 46 – 47 mm | 47 | 45.5 – 50.0 |
| 1-1/2 in | about 1.89 in / 48.0 mm (calculated) | 48 – 49 mm | 47 (50 if over 49 mm) | 45.5 – 50.0 / 48.5 – 53.0 |
| 1-3/4 in | 2.14 in / 54.4 mm (Gates 4175) | 54.5 – 55.5 mm | 55 | 53.5 – 58.0 |
| 2 in | 2.42 in / 61.5 mm (Gates 4175) | 61.5 – 62.5 mm | 60 (listed 60.6) | 58.5 – 64.0 |
Watch out: wall thickness varies by brand and hose type. Silicone, molded OEM and thin-wall heater hose can sit 1 to 3 mm off these figures, often the entire window. Use the sheet to narrow to two sizes, then measure.
For the 5/8 heater hose question specifically: the 0.94 in (23.9 mm) Gates figure lands in both the 24 mm and 25 mm windows once the hose is on a spigot. A 22 mm clamp (window to 24.2 mm) is the common wrong pick, because 5/8 in converts to 15.9 mm and people forget the wall. For clamp choice by hose duty rather than by size, see our guide to the best hose clamps for radiator hoses.
How to measure for a spring clamp (not like a worm clamp)
Our guide on how to measure hose clamp size covers the basic OD-on-the-fitting rule. Spring clamps need a stricter routine, because a 1 mm error can take you out of the window.
1. Measure the hose OD installed, at the clamp position
Push the hose fully onto its spigot and measure exactly where the clamp will sit, usually just behind the bead. A hose stretched over a bead can be 1 to 2 mm larger there than 10 mm along. Take two readings at 90 degrees and use the larger, because hose ovalizes.
No calipers? Wrap a paper strip around the hose, mark the overlap and divide the length by pi: a 116 mm strip means about 36.9 mm OD.
2. Measure cold
Rubber hose swells when hot and under pressure. Measure a warm engine and you may pick a clamp that is loose once things cool. Size at the cold condition and let the spring follow the swell.
3. Read the old clamp’s free ID, relaxed, not expanded
If you are replacing an original clamp, it is your best clue. Remove it, let it close fully on the bench, and measure the inside diameter across the band with the tabs at 3 and 9 o’clock. Compare that reading to the free / closed ID column in the chart, not the nominal column.
Rotor Clip’s CTB-29 data page notes that its free diameter is measured only after the clamp has been expanded fully once. That matters: a brand new clamp often relaxes slightly larger after its first full opening, so an old clamp that has been in service is already at its settled free size. A used clamp that measures 34.3 to 34.7 mm relaxed is consistent with a 38 mm part in the table; one at 32.5 mm points to 36 mm.
Caution: an overheated or corroded clamp can lose spring and measure larger than new. Record all readings in mm to one decimal; spring clamp catalogs are metric even in the US.
4. Check band width against the hose bead and spigot length
Band width (commonly 8.4, 10, 12 or 15 mm) has to fit the land between the hose end and the spigot bead. A 15 mm band on a short heater pipe may ride onto the bead. You want the full band on hose over solid spigot, with a millimeter or two to spare.
Four worked examples: where the hose OD sits in the window
The simplest way to compare two candidate sizes is to work out how far into the window your OD sits, as a percentage:
Window position (%) = (installed OD − window minimum) ÷ (window maximum − window minimum) × 100
Rule of thumb: aim for roughly 25 to 75 percent. Below about 25 percent, a little hose compression set or a cold morning takes you toward the minimum functional diameter. Above about 75 percent, the clamp is hard to fit and has little reserve for a swelling hose. This band is our working guidance, not a maker specification; the maker only promises the full window.
Example 1: 5/8 in heater hose, measured 24.6 mm
- 24 mm clamp, window 23.3 to 26.0 mm (width 2.7): (24.6 − 23.3) ÷ 2.7 = 48 percent.
- 25 mm clamp, window 24.0 to 26.8 mm (width 2.8): (24.6 − 24.0) ÷ 2.8 = 21 percent.
- 22 mm clamp, window 21.3 to 24.2 mm: 24.6 is above the maximum. Overstressed, reject.
Pick: the 24 mm clamp, right in the middle. The 25 mm part would work but sits near its lower edge.
Example 2: 1 in radiator hose, measured 37.2 mm
- 36 mm clamp, window 35.0 to 39.0 mm (width 4.0): (37.2 − 35.0) ÷ 4.0 = 55 percent.
- 38 mm clamp, window 37.0 to 41.5 mm (width 4.5): (37.2 − 37.0) ÷ 4.5 = 4 percent.
Pick: the 36 mm clamp. The 38 mm part is inside its window but only 0.2 mm above minimum. This is the classic “went on easily, now it weeps” mistake: choosing the stamped number closest to the measured OD, when that number sits at the bottom of the window.
Example 3: 1-3/4 in lower hose, measured 52.9 mm
- 50 mm clamp, window 48.5 to 53.0 mm (width 4.5): (52.9 − 48.5) ÷ 4.5 = 98 percent. Almost fully opened.
- 55 mm clamp, window 53.5 to 58.0 mm: 52.9 is 0.6 mm below the minimum. Too loose.
Pick: neither. The OD falls in a gap between catalog sizes. Re-measure (0.6 mm is an easy error on a 53 mm hose), look for an in-between nominal and check its window, or switch to a screw-adjusted constant tension clamp. A 52.9 mm hose sits inside the 1.56 to 2.50 in (40 to 64 mm) range of a constant torque clamp such as OneTouch’s 4CT0032.
Example 4: same nominal, two makers, measured 30.2 mm
- DIN-style 29 mm (EFC table), window 28.0 to 31.5 mm (width 3.5): (30.2 − 28.0) ÷ 3.5 = 63 percent.
- Rotor Clip CTB-29, clamping range 28.8 to 30.5 mm (width 1.7): (30.2 − 28.8) ÷ 1.7 = 82 percent.
- Rotor Clip CTB-30, clamping range 29.8 to 31.5 mm (width 1.7): (30.2 − 29.8) ÷ 1.7 = 24 percent.
Lesson: “29 mm” is not one window. The DIN-style part suits this hose well; in the Rotor Clip family the 30 is the better match, and the 29 sits high. When you cross from one brand to another, compare windows, never just the stamped number.
Signs you picked the wrong spring clamp size after fitting
A spring band hose clamp size chart gets you close; the hose tells you the rest within a heat cycle or two.
Clamp too big (OD below or near the bottom of the window)
- The clamp slides along the hose by hand once installed, or rotates freely with light finger pressure.
- Coolant weeps or crusts at the hose end after a cold start, then dries when warm (the hose swells and seals hot, leaks cold).
- The tabs sit close together, nearly at their free gap, with the clamp on the hose.
Clamp too small (OD at or above the top of the window)
- You struggle to open it far enough to pass over the hose, and the band visibly flexes into an oval when released.
- The band bites a deep, sharp-edged groove into the hose cover, or the hose bulges on either side of the band.
- The tabs are very far apart and the band does not sit round on the hose.
- After removal, the clamp’s free ID has grown compared with a new one of the same part: it has taken a set from overstress.
- Cracks start at the tab bends or at the band window, sometimes months later.
Right size, wrong position or width
- One edge of the band is on the spigot bead, so the band tilts and grips only on one side.
- The band overhangs the hose end, pinching the last few millimeters of rubber with no spigot behind it.
Any “too small” sign is a replace-now job: a cracked band can let go suddenly. For background on where these parts typically fail, see what a spring band hose clamp is and its weak spots. If corrosion rather than size is your problem, our comparison of 304 vs 316 spring band hose clamps covers why stainless spring temper behaves differently from carbon spring steel.
When to stop using spring clamps and switch
Spring band clamps excel on the joint they were designed for and struggle when the joint changes. Consider switching when:
- Your measured OD falls between catalog windows, as in Example 3, or you cannot measure reliably enough to hit a 2 to 4 mm window.
- You are changing hose brands or types, such as moving from an OEM molded hose to a universal or silicone hose with a different wall. The old clamp size no longer follows.
- You stock a mixed fleet and would need dozens of nominal sizes on the shelf.
- The joint is larger than the catalog range, roughly beyond 90 mm for DIN 3021 Type A in a 12 mm band, and beyond 67 mm for a 15 mm band.
Our spring hose clamp vs worm gear hose clamp comparison covers grip, heat cycling and serviceability. The usual middle ground is a screw-adjusted constant tension clamp: a worm drive with a stack of conical spring washers that keeps load on the hose as it shrinks and swells, but with a range measured in inches rather than millimeters. The benefits and typical uses are in stainless steel constant tension hose clamps.
OneTouch does not stock spring band clamps. We supply constant tension (constant torque) clamps as the adjustable, spring-loaded alternative. For example, the 4CT0016 constant torque clamp covers 13/16 to 1-1/2 in (21 to 38 mm) with a 9/16 in 301 stainless band, a 410 stainless screw, a 5/16 in hex and a 70 in-lb rating. That one part covers the hose ODs served by roughly eight spring clamp sizes in our chart (22 to 35 mm nominal). The 4CT0024 constant torque clamp covers 1-1/16 to 2 in (27 to 51 mm), which takes in the 3/4 through 1-1/2 in coolant hoses on the swap sheet.
Sizing coiled wire clamps on the same job? The companion spiral hose clamp size chart applies the same measuring discipline to that type.
Spring band hose clamp size chart FAQ
What does the number on a spring band clamp mean?
It is the nominal diameter in millimeters. In DIN-style catalogs it sits near the bottom of the working window, so a 38 mm clamp in the EFC and Würth data works on hose ODs from 37.0 to 41.5 mm. It is not the free size (34.5 mm for that part) and not the midpoint of the range.
What size spring clamp fits a 5/8 heater hose?
A Gates 5/8 in heater hose lists 0.94 in (23.9 mm) OD, so once it is on a spigot a 24 mm or 25 mm nominal spring clamp usually fits. Measure the fitted hose: if it reads 24.0 to 25.5 mm, the 24 mm part (23.3 to 26.0 mm window) sits closest to mid-window. Thicker-wall hose may need a 25 or 27 mm part.
Should I measure the hose ID or OD for a spring clamp?
The OD, installed on the spigot, at the spot where the clamp will sit, with the hose cold. The ID only helps you estimate. Spring clamp windows are 2 to 5 mm wide, so a guess from the ID plus an assumed wall is often off by enough to miss.
Can I use a spring clamp one size smaller for a tighter grip?
Only if your hose OD still falls inside the smaller part’s window. If it is above that part’s fully opened diameter, the band is overstressed, may crack at the tabs and can take a permanent set. Use the window position formula and stay roughly between 25 and 75 percent.
What band widths do spring hose clamps come in?
DIN 3021 Type A lists 12 mm and 15 mm bands. Catalogs also offer narrower 8.4 mm and 10 mm families, and Rotor Clip lists a 15 mm CTB and a 12 mm CTN. Choose the width that fits fully on the hose land behind the spigot bead.
How do I size a replacement from the old spring clamp?
Take it off, let it close fully and measure the relaxed inside diameter. Compare that with the free or closed diameter column in a maker’s chart, then confirm by measuring the fitted hose OD. Read the stamped number too, but trust measurements if the old clamp is corroded or has been overheated.
Are spring clamp sizes the same across brands?
Not exactly. Rotor Clip’s CTB-29 lists a 28.8 to 30.5 mm clamping range, while a DIN-style 29 mm part runs 28.0 to 31.5 mm. Compare the published windows when you swap brands rather than matching the stamped number.
How do spring clamp sizes convert to SAE worm clamp sizes?
There is no direct conversion, because an SAE worm size covers a wide range and a spring clamp covers a narrow one. Convert your hose OD instead, then look it up in each chart. Our hose clamp size chart in mm and inches lists the worm gear ranges.
Need an adjustable alternative to a spring band clamp?
Use the spring band hose clamp size chart above to find the window your hose needs. If it lands cleanly in one size, buy that spring clamp from a specialist. If it falls between sizes, or you are tired of stocking 25 nominal diameters, browse our constant tension hose clamps, which cover each range with a single screw-adjusted, spring-loaded part.
Have another maker’s part number? Run it through the OneTouch hose clamp cross-reference tool. For volume pricing or export shipping, send your list via the request a quote page and we will reply with sizes, pack quantities and lead times.