Free building-performance calculator

P/A Ratio Calculator

Calculate the exposed perimeter-to-floor-area ratio used when assessing the thermal performance of a ground floor.

P ÷ A Internal exposed perimeter ÷ floor area

Plan-based guidance

Measure the internal exposed edge

Example plan showing the internal exposed perimeter The internal faces of the top, left and bottom external walls are highlighted in amber. The right party wall adjoining a heated building is dashed grey and excluded. Room Room Room Room Internal face next to outside — include Adjoining heated space Exclude Floor area measured internally
01

Calculate your ratio

Enter the floor measurements

m
Only the internal wall faces adjoining outside or an unheated space.
m²
Measure the relevant heated floor area to the internal faces of perimeter walls.

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Calculation guidance is based on the simplified solid ground-floor approach in BS EN ISO 13370. England benchmark information is referenced from Approved Document L, Volume 1.