MEASUREMENTS AND PURCHASE DECISIONS
Measure circular arc rise from its chord, not from the centre.
Where should the rise measurement begin when inferring radius?
Use the matching calculator
Use this decision sequence
- Join the two arc endpoints with a straight chord.
- Measure the maximum perpendicular rise from that chord to the minor arc.
- Use the minor-arc tool only when rise is no more than half the chord.
- Check the circular assumption before treating the inferred radius as a cutting dimension.
Keep the quantities distinct
| Quantity or assumption | How to use it |
|---|---|
| Chord width | Straight endpoint-to-endpoint distance. |
| Arc rise | Perpendicular distance from chord to arc. |
| Radius | Distance from circle centre to arc, usually not measured by the same line. |
Worked comparison
An illustrative 2 m chord with 1 m rise is a semicircle of radius 1 m: R = 2²/(8×1)+1/2 = 1. A shallower 0.25 m rise over the same chord gives R = 2²/(8×0.25)+0.25/2 = 2.125 m. The same chord can therefore belong to very different radii.
Check before using the estimate
This is circular geometry only; it does not determine an arch’s structural behaviour.
All dimensions, product properties, prices and specifications in this example are illustrative arithmetic inputs. Use the values from your measured plan, selected product data sheet and supplier quote. This guide does not choose construction specifications or certify safety.
Calculation and scope checked 2026-10-04. Methods and scope.