Estimate rifle chamber pressure conversions between CUP and PSI in both directions, including kCUP, kpsi, MPa, and bar equivalents.
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CUP to PSI Formula
The calculator uses the widely cited empirical relationship between copper crusher (CUP) and piezoelectric (PSI) pressure measurements for centerfire rifle cartridges.
PSI = 1.516 * CUP - 17902
For the reverse mode:
CUP = (PSI + 17902) / 1.516
- PSI — pressure measured by a piezoelectric transducer, in pounds per square inch
- CUP — pressure measured by the copper crusher method, in copper units of pressure
- 1.516 — slope from the empirical fit (Denton bridge fit, SAAMI rifle data)
- 17,902 — intercept from the same fit, in psi
CUP and PSI are not the same physical quantity. They come from two different test methods, so any conversion is a statistical estimate, not an exact equivalence. The fit was developed for full-power rifle cartridges in the roughly 28,000 to 54,000 CUP range. Outside that band, accuracy drops. The formula does not apply to handgun, shotgun, or black powder pressures, which use different test setups and different relationships.
Reference Values
Approximate CUP and PSI values for common rifle cartridges, based on published SAAMI maximum average pressures.
| Cartridge | CUP (max) | PSI (max) |
|---|---|---|
| .223 Remington | 52,000 | 55,000 |
| .243 Winchester | 52,000 | 60,000 |
| .270 Winchester | 52,000 | 65,000 |
| .308 Winchester | 52,000 | 62,000 |
| .30-06 Springfield | 50,000 | 60,000 |
| .30-30 Winchester | 38,000 | 42,000 |
| 7mm Remington Magnum | 52,000 | 61,000 |
| .300 Winchester Magnum | 54,000 | 64,000 |
Pressure unit conversions for cross-reference:
| Unit | Equals |
|---|---|
| 1 kpsi | 1,000 psi |
| 1 MPa | 145.038 psi |
| 1 bar | 14.504 psi |
| 1 kCUP | 1,000 CUP |
Worked Example and Notes
Example: A load tests at 50,000 CUP. Plug into the formula:
PSI = 1.516 × 50,000 − 17,902 = 75,800 − 17,902 = 57,898 psi
Why the answer is an estimate. The CUP method measures the deformation of a copper slug, which integrates pressure over time. The PSI method uses a piezoelectric transducer that captures peak pressure directly. The two readings correlate but do not match. The 1.516 / 17,902 fit is an average across many cartridges; individual cartridges can deviate by several thousand psi.
Do not use this conversion to approve handloads. Refer to a current load manual for the specific cartridge and pressure system. The calculator is for cross-referencing published data, not for substituting one pressure spec for another in a load workup.
Why is there a negative intercept? The line is a regression fit, not a physical zero point. At very low CUP values, the formula produces unrealistic results, which is why the calculator rejects inputs that would yield zero or negative PSI.
