Proof load is the tension a threaded fastener can hold without taking permanent deformation: Fp = At × Sp, the tensile stress area times the proof strength of the property class or grade. Proof strength sits a deliberate notch below yield — 580 MPa proof against 640 MPa yield for class 8.8 through M16 (larger 8.8 bolts step up to 600 MPa proof), and the systems align across the ocean: SAE Grade 5 proof is 85 ksi, about 586 MPa, against 8.8’s 580. An M12 class 8.8 with its 84.3 mm² stress area has a proof load of 84.3 × 580 = 48,900 N.
Its practical role is as the anchor for preload targets. Torque charts and calculators set the clamping force as a fraction of proof load — 75% is the standard for reusable joints, with permanent single-use joints sometimes taken to 85–90% — because anchoring on proof rather than yield leaves margin twice over: below proof, and further below yield, keeping the bolt elastic through installation scatter and service loading. The 75%-of-proof M12 lands at 36,700 N of preload and 435 MPa of steady tension, about 68% of yield, before the joint ever sees an external load.