How to measure
a flat gasket correctly —
OD, ID and thickness
Measurement errors in any of the three dimensions can cause sealing problems — though how quickly and how severely depends on the fitting geometry and the margin of error. Here is how to take each measurement correctly — from a recovered gasket, from a worn one, or from the fitting itself when the original is gone.
The three measurements
The outside diameter is the total width of the gasket across its widest point. This dimension must fit within the fitting recess — the raised face or step on the union or fitting body where the gasket seats. A gasket OD that is too large for the recess will fold or buckle at the edge as the joint is tightened. A gasket OD that is significantly smaller than the recess will sit off-centre and seal unevenly.
Measure across the full outer edge of the gasket with a digital calliper, placing the jaws at the widest point. For a used gasket, OD does not change significantly in service — the measurement from a recovered gasket is reliable.
The inside diameter is the width of the central hole. This must be large enough to clear the bore through-hole in the fitting — the gasket must not obstruct flow. It must also be small enough to sit properly on the sealing face rather than hanging unsupported over the opening.
Measure across the inner hole with the inside jaws of a digital calliper. For a used gasket, ID is also stable in service — measure normally. When measuring from the fitting, the ID to use is the bore diameter of the through-hole, with the gasket ID matching or slightly exceeding this.
Thickness affects how the gasket compresses in the joint. A gasket significantly outside the compression range the fitting is designed for may not seal correctly — the precise effect depends on fitting geometry, material and design.
For a new gasket, take several thickness readings with a suitable low-force gauge. A used gasket may be compressed across all or only part of its face, so no universal measurement position recovers the original thickness. Record the thickest consistent undamaged area as a clue, then verify the nominal thickness from equipment data, a confirmed part reference or the joint's recess, stand-off and required compression.
Measuring a worn or damaged gasket
Never reuse a recovered gasket. Measure it to find the replacement size — then discard it. A gasket removed from service has permanently deformed under compressive load. It will not seal reliably on reassembly regardless of how intact it appears.
When measuring a used gasket to determine the replacement size:
- OD: measure normally across the outer edge. OD is stable in service.
- ID: measure normally across the inner hole. ID is also stable in service.
- Thickness: take several readings and map them to the witness pattern. Do not assume any edge was unconstrained; confirm original thickness from a drawing, part record or verified joint geometry.
- Condition check: if the gasket is cracked or delaminated, OD and ID measurements may still be useful as a reference — treat them as approximate starting points rather than confirmed dimensions. For thickness, cross-check the fitting recess depth against the BSP size chart — recess depth is a starting reference only.
Measuring from the fitting — when the original gasket is gone
When the original gasket is missing, damaged beyond measurement, or the OEM part number is obsolete, measure the fitting recess directly.
- Recess OD: the inside diameter of the raised face or step where the gasket sits. Use this as the reference for gasket OD.
- Bore ID: the diameter of the through-hole. The gasket ID must clear this without restricting flow.
- Recess depth: a rough starting guide for thickness only — fitting design and compression behaviour also determine the correct thickness. Use it to narrow the range, then cross-reference against the BSP size chart.
- Thread OD: measure the male thread OD with the calliper to confirm the BSP thread size. This cross-references your recess measurements against known standard dimensions.
Cross-reference — common BSP sizes
Once you have your measurements, cross-reference against common BSP flat gasket dimensions to confirm the thread size and select the correct variant:
| BSP size | Thread OD | Gasket OD | Gasket ID | Thickness | Common application |
|---|---|---|---|---|---|
| 3/8″ | 16.66 mm | 14.5 mm | 8–12 mm | 2 or 3 mm | Instrument fittings, small valves |
| 1/2″ | 20.95 mm | 18.5 mm | 10–14 mm | 2 or 3 mm | Filling loop, PRV, 15 mm pipework |
| 3/4″ | 26.44 mm | 24 mm | 14 or 16 mm | 2 or 2.5 mm | Pump unions, heat exchangers |
| 1″ | 33.25 mm | 30 mm | 21–24 mm | 2 mm | Larger pumps, 28 mm pipework |
| 11/4″ | 41.91 mm | 39 mm | 30 mm | 2 mm | Commercial connections |
| 11/2″ | 47.80 mm | 44 mm | 36 mm | 2 mm | Large pump unions, commercial |
Gasket dimensions are indicative for standard BSPP flat-face fittings. Always verify against the actual fitting recess. ID varies between fitting manufacturers for the same nominal thread size — confirm separately.
The 3/4 inch BSP gasket has three standard variants: 24×14×2.5 mm, 24×16×2 mm and 24×16×2.5 mm. Two different inside diameters — 14 mm and 16 mm — for the same outside diameter. This is why measuring ID separately matters. Specifying only "3/4 inch BSP gasket" without confirming ID is a common source of sizing errors for pump union replacement.
What you need to measure
A digital calliper is the right tool. Analogue callipers work but are slower to read. A ruler is not adequate for gasket measurement — the tolerances involved are tighter than a ruler can reliably resolve.
A digital calliper with 0.01 mm resolution is sufficient for all standard gasket sizing. Measure each dimension twice and confirm consistent readings before ordering. For thickness on a used gasket, measure at several points around the outer edge — away from the contact zone — and use those readings as a guide to the nominal thickness.
OD, ID and thickness — but first identify the joint geometry, and do not reconstruct nominal thickness from one point on a used gasket.
If the original gasket is available — measure it, note all three dimensions, then discard it. If only the fitting is available — measure the recess OD, bore ID and recess depth, cross-reference against the BSP size chart, and confirm thread OD to verify the nominal size. Three numbers. One calliper. The rest is selection.
FAQ
How do I know what size flat gasket I need?
Three measurements determine the correct flat gasket: outside diameter (OD), inside diameter (ID) and thickness. Measure OD and ID with a calliper from the original gasket if available. A used gasket does not reliably reveal its original thickness. Measure several areas and use any clearly uncompressed projecting edge only as a secondary clue; verify nominal thickness from equipment data or the joint geometry. If the original gasket is unavailable, measure the fitting recess: recess OD gives a starting reference for gasket OD, bore diameter gives a starting reference for gasket ID, and recess depth is a rough starting guide for thickness only.
Can I measure a used gasket to find the replacement size?
Yes, with one important caveat. A used flat gasket has been compressed at the sealing contact ring and is permanently thinner there than at the outer edge. Measure OD and ID normally — these usually change less than thickness in service. For thickness, record several readings, but do not assume the outer edge was unloaded in every joint. Verify the original nominal thickness from equipment data or the joint geometry.
What if I do not have the original gasket to measure?
Measure the fitting recess directly with a calliper. The recess OD — the inside diameter of the raised face or step where the gasket seats — gives a reference for gasket OD. The bore through-hole diameter gives a reference for gasket ID. Recess depth is a rough starting indication for thickness only — fitting design and compression behaviour also affect the correct choice. Cross-reference your measurements against a BSP size chart to confirm the nominal thread size.