Tape Sag Correction Calculator

Work out tape sag correction instantly with clear inputs, formula shown and shareable results.

About Tape Sag Correction Calculator

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.

Formulas

Sag correction

Correction = -w² L³ / (24 P²), w in N/m and P in N

Mid span sag

Sag = w L² / (8 P)

Worked example

With the default values below, Tape Sag Correction Calculator returns Sag correction of -0.004328. Change any input above and every figure here is recalculated in your browser.

Inputs used

Unsupported span lengthminimum 0.1 m: 30 m
Mass of the tape per metreminimum 0.001 kg/m: 0.02 kg/m
Applied tensionminimum 1 N: 100 N

Results

Sag correction: -0.004328 m
Corrected span length: 29.99567 m
Correction in millimetres: -4.328 mm
Sag at mid span: 0.2206 m

Step by step

1Values used
2Sag correction
3Mid span sag
4Sag correction= -0.004328 m
5Corrected span length= 29.99567 m
6Correction in millimetres= -4.328 mm
7Sag at mid span= 0.2206 m

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 6 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Tape Sag Correction figure worked out?

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.

Can sag be eliminated by tension alone?

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.

What values do I need to enter?

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.