Free calculator
Tributary area calculator
Work out the line load a floor puts on a wall or beam. Enter the spans either side and your floor loads. The result updates as you type, with no sign-up.
m
m
kN/m²
Floor build-up plus ceiling and finishes. Typed by you.
kN/m²
Preset:
m
Adds total load in kN.
Result
Loaded width
3.50 m
Characteristic line load (gk + qk)
10.50 kN/m
ULS line load (1.35gk + 1.5qk)
14.96 kN/m
How it works
A one-way floor spans between parallel supports. Each support carries half of the floor on each side of it, so the loaded width is half of span a plus half of span b. For an edge support there is only one side, so b is zero.
Loaded width = a / 2 + b / 2
Characteristic = (gk + qk) × loaded width
ULS = (1.35 gk + 1.5 qk) × loaded width
Each span is treated as simply supported, so continuity over the support is ignored. A continuous floor puts more load on an internal support than this, so treat the result as a first check. The 1.5 kN/m² preset is the domestic floor imposed load from the UK National Annex to BS EN 1991-1-1. Dead load depends on your build-up, so enter your own figure.
Worked example
An internal wall with a 4 m span on one side and 3 m on the other. Dead load 1.5 kN/m², imposed load 1.5 kN/m².
Loaded width = 4 / 2 + 3 / 2 = 3.5 m
Characteristic = (1.5 + 1.5) × 3.5 = 10.5 kN/m
ULS = (1.35 × 1.5 + 1.5 × 1.5) × 3.5 = 4.275 × 3.5 = 14.96 kN/m
Over a 5 m long wall that is 52.5 kN characteristic and 74.8 kN at ULS.
Check the result
This calculator is for checking. Results must be verified by a competent engineer before they are used in a design. It does not include wall self weight, point loads, openings or load from floors above.
Read more: how loaded width works and tributary regions.