Floor Joist Calculator (UK): Joists, Strutting and Boards

Enter the length of the room along which the joists are spaced, the span of each joist and the centres. You get the number of joists, their cut length and the stock length to order, the strutting from Approved Document A, and the flooring boards.

UK edition. Switch to the US edition for feet, inches and US gallons.

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Example job shown. Change the numbers to match yours.

Joists

14 joists

A joist at each end and one every 400 mm

Cut length of each joistThe span plus the bearing at both ends: 3.80 m
Stock length to orderThe next length in 300 mm steps: 3.9 m
Timber to buyAll the joists, in the stock length: 54.6 m

Strutting

Rows of struttingOver 2.5 m up to 4.5 m: one row at the middle: 1 rows
Struts (between the joists)One between each pair of joists in each row: 13 struts

Flooring

Flooring boards of 2400 × 600 mmWith 10% extra: 14 boards

The count is the length divided by the centres, plus one for the last joist. Joists over 2.5 m up to 4.5 m need a row of strutting in the middle, and joists over 4.5 m need two rows at the third points (Approved Document A). The joist size depends on the span, the spacing, the timber grade and the load, and comes from a span table, a design or a building control officer: it is not on this page. Add trimmers and double joists around stairwells, chimneys and loft hatches, and a joist under any wall that runs parallel to the joists.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

How this calculator works

Worked example: a floor 5 m long with joists at 400 mm centres needs 5 ÷ 0.4 = 12.5, so 13, plus one: 14 joists. Each spans 3.6 m and sits 100 mm on each wall, so it is cut at 3.80 m, and the stock length is 3.9 m. That is 54.6 m of timber. A span over 2.5 m takes 1 row of strutting (13 struts). At 450 mm it is 13 joists, and at 600 mm 10. A span of 5 m needs 2 rows, and 2.4 m needs 0. The boards are 14 sheets of 2400 × 600 mm.

Joists for a 5 m length at each spacing, and the strutting for different spans

Joists across 5 m, and the rows of strutting for the span.

Span	Joists at 400 mm	Joists at 450 mm	Joists at 600 mm	Strutting rows
2.4 m	14	13	10	0
3 m	14	13	10	1
3.6 m	14	13	10	1
4.2 m	14	13	10	1
4.8 m	14	13	10	2
5.4 m	14	13	10	2

Tips before you buy

Choose the joist size from a span table for your timber grade, spacing and load, or ask the building control officer.
Start the spacing from one wall, and check that the sheets of flooring join on the middle of a joist.
Double up the joists under a wall that runs the same way, and around stairwells and hatches.
Fit the strutting after the joists are level, and fix it tight so that the joists work together.
Notch and drill joists only in the places allowed, in the middle of the depth and away from the supports.

Tools & materials for this project

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Joist hangers Flooring boards 22 mm Strutting Floor screws Pipe and cable detector

Frequently asked questions

How many joists do I need?

The length of the floor divided by the centres, rounded up, plus one: 14 joists for 5 m at 400 mm centres.

How far apart should floor joists be?

Usually 400, 450 or 600 mm centre to centre. The spacing goes with the size of the joist, the span and the flooring thickness.

When do I need strutting?

Joists over 2.5 m up to 4.5 m need one row of strutting in the middle, and joists over 4.5 m need two rows at the third points, in Approved Document A.

What size joist do I need?

It depends on the span, the spacing, the timber grade and the load. Use a span table or ask your building control officer or an engineer: the size is not on this page.

How many floor boards do I need?

14 boards of 2400 × 600 mm for this floor with 10% extra.

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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.

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