What this calculator works out — and what it does not
This calculator works out how many timber members are needed across a span at a given spacing, and the total linear metres to buy including waste.
Despite the name, it does not tell you what section of timber to use. Deciding that a joist should be 47 by 195 mm rather than 47 by 145 mm is a structural calculation based on span, load, species and grade, and it is covered by span tables and building regulations rather than by a quantity tool.
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Before you order
- Get the section size from a span table or a structural engineer first. TRADA and the Approved Documents publish tables for common domestic cases.
- Spacing centres are typically 400 or 600 mm for joists and rafters, but the correct figure comes from the span table alongside the section size.
- Member length should be the full length you need to buy, allowing for bearing at each end.
- Timber is sold in fixed lengths. Ordering 36.3 linear metres means buying whole lengths that cover it, usually with more offcut than the waste allowance suggests.
A span table gives combinations, not independent choices. The same timber section will span further at 400 mm centres than at 600 mm, and choosing a wider spacing to save timber may push the section up a size and cost more. Read them as pairs.
How this calculator works
A member count across the span, then a length total:
Members = (span in mm ÷ spacing) rounded down, plus 1
Linear metres = members × member length × (1 + waste ÷ 100)The plus one accounts for the member at both ends of the run — ten spaces need eleven members.
The calculation ignores trimmers, noggins, doubled joists around openings and any additional members at the perimeter, all of which add to a real order.
Worked example: a 4 m span at 400 mm centres
Using the default figures — a 4 m span, 3 m members at 400 mm centres with a 10% allowance:
- Members: (4,000 ÷ 400) + 1 = 11
- Linear metres including waste: 11 × 3 × 1.1 = 36.3 m
In practice you would buy eleven 3 m lengths, which is 33 m, and the waste allowance covers cutting rather than adding a twelfth. If the merchant stocks 3.6 m lengths and not 3 m, the real order becomes 39.6 m with 6.6 m of offcut — which is why checking stocked lengths before finalising a design saves both money and skip space.
Common mistakes
- Using this to choose a section size. It counts members; it does not size them.
- Forgetting the plus one. A run needs a member at each end.
- Ignoring stocked lengths. Buying 3 m lengths when the merchant stocks 3.6 m changes the waste substantially.
- Omitting noggins and trimmers. They are extra and can add 10 to 15%.
- Assuming any timber will do. Structural work needs graded timber, marked C16 or C24.
Frequently asked questions
How do I find the right joist size?
Span tables. The Approved Documents supporting building regulations include tables for common domestic floor and roof cases, and TRADA publishes more detailed guidance. The table needs the span, the spacing, the load and the timber grade. For anything outside the standard cases — long spans, unusual loads, altered structure — a structural engineer is the right route.
What do C16 and C24 mean?
They are strength grades for structural softwood under BS EN 338. C24 is stronger and stiffer than C16 and can span further at the same section, which sometimes makes it cheaper overall despite the higher price per metre. Structural timber must be graded and marked; ungraded timber cannot be used for structural work.
Do I need building regulations approval?
For anything structural, generally yes. Cutting into or replacing joists, forming an opening, altering a roof structure or converting a loft all engage building regulations. Notifying and having the work inspected also protects you at sale, when a buyer's solicitor asks for completion certificates.
How much extra should I allow?
10% covers cutting waste on a simple job. Add more for complex layouts, and remember noggins, trimmers and doubled members around openings are additional items rather than part of the allowance. Ordering slightly long is usually cheaper than a second delivery.
Is what I enter stored?
No. Spans and spacings are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Approved Document A on structure, and span table guidance
- Health and Safety Executive — safe use of power saws and manual handling of timber
Sources are checked at publication and can change — how I choose and check references.