Work out tape sag correction instantly with clear inputs, formula shown and shareable results.
A tape suspended between supports hangs in a catenary, so the horizontal distance is shorter than the tape length. The correction is always negative and equals w²L³ / (24P²), where w is weight per metre and P is tension. Because span length is cubed and tension is squared, supporting the tape or pulling harder both help dramatically.
Sag correction
Correction = -w² L³ / (24 P²), w in N/m and P in N
Mid span sag
Sag = w L² / (8 P)
Correction = -w² L³ / (24 P²), w in N/m and P in N. A tape suspended between supports hangs in a catenary, so the horizontal distance is shorter than the tape length.
Not fully, and over-tensioning stretches the tape. The normal tension is the pull at which the sag and elongation corrections cancel, which is why precise taping specifies a standard pull.
This calculator takes 3 inputs: Unsupported span length, Mass of the tape per metre, Applied tension. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.