Floor Joist Span Calculator UK
Getting a floor joist size right is the difference between a solid floor and one that bounces every time someone crosses the room. This UK floor joist span calculator works both ways: tell it your joist size, timber grade and spacing and it returns the maximum clear span allowed; or tell it the span you need to cover and it returns the smallest joist that will do the job. It's built around the span tables used for UK domestic floors, works in metric, and runs on your phone on site. It assumes a standard domestic floor — an imposed load of 1.5 kN/m² and a dead load up to 0.50 kN/m² — which covers most timber floors with a chipboard or T&G deck and a plasterboard ceiling.
C16 is the standard UK carcassing grade sold in most merchants. C24 is graded stronger and stiffer and spans further for the same size. If your timber is stamped, the grade is on the stamp.
The centre-to-centre gap between joists. 400 mm is the most common in UK homes; 600 mm suits some engineered layouts but spans less far.
Regularised/CLS carcassing is 47 mm wide. Pick the depth you have or plan to buy. Deeper joists span further — depth does far more work than width.
Spans are calculated on the finished (regularised) section — a nominal 47 × 200 mm joist finishes about 47 × 195 mm, which is the size the span table uses.
Enter your joist details to see the maximum span.
Spans follow the abridged Building Regulations Table A1 (NHBC / TRADA) for domestic floor joists: imposed load 1.5 kN/m², dead load 0.25–0.50 kN/m² (excluding the joist's own weight), Service Class 1 or 2. The table makes no allowance for a partition, tank, trimmer or other concentrated or line load bearing on the joists — those need a structural engineer.
| Nominal size (47 × depth) | Typical use | Grade usually stocked |
|---|---|---|
| 47 × 100 mm | Short spans, small outbuildings | C16 |
| 47 × 150 mm | Common landing / small room joist | C16 |
| 47 × 175 mm | Standard bedroom floor | C16 / C24 |
| 47 × 200 mm | Larger rooms, longer clear spans | C16 / C24 |
| 47 × 225 mm | Long spans, heavier floors | C24 |
47 × 175 and 47 × 200 mm cover the bulk of UK domestic floors — start there and adjust.
How to use this calculator
- Pick the mode — do you know the joist and want the span, or know the span and want the joist?
- Set the grade. If you don't know, assume C16 — it's what most merchants sell as standard carcassing, and assuming the weaker grade keeps you safe.
- Set the spacing to match your layout — 400 mm centres is the UK default.
- Read the result as an indicative maximum, then confirm it with Building Control before you order timber.
When you're unsure of the grade, pick C16 — designing to the weaker timber never leaves you short.
What decides how far a joist can span
Depth matters far more than width. The span a joist can reach scales roughly with its depth, while stiffness scales with the depth cubed — going from 150 mm to 200 mm deep doesn't just add a little capacity, it transforms the floor. Width at 47 mm is standard across all carcassing sizes and contributes much less.
Grade lifts span by 10–15% for the same size. C24 timber is graded to be both stronger and stiffer than C16, so a 47 × 200 mm C24 joist spans 4.35 m at 400 mm centres against 4.04 m in C16. The difference is worth having when joist depth is tight against a low ceiling.
Spacing adds a useful margin but doesn't transform the span. Going from 600 mm to 400 mm centres doesn't double the span — it adds a buffer, often enough to avoid stepping up a joist size. The floor loading is the fixed variable: the 1.5 kN/m² imposed figure comes from BS EN 1991-1-1 (the loading Eurocode) and is the standard for domestic floors. Heavier uses — a bath, a stud wall running along the joists, a solid stone floor — push beyond this figure and outside the table's assumptions.
Reach for a deeper joist before a wider or closer-spaced one — depth buys the most span per pound.
Deflection, not strength, is usually the limit
Most pages on this topic focus on whether a joist will hold the weight. That's not usually the real question. A domestic floor joist almost never fails on bending strength first — it hits the deflection limit, which is a different test.
Eurocode 5 (BS EN 1995-1-1) caps deflection to keep floors from feeling bouncy, commonly around span/333 or 14 mm maximum. A joist can be strong enough to carry the load and still deflect enough to make a floor feel spongy and unacceptable. That's why the span table figures are lower than a pure strength calculation would suggest — the tables already build the stiffness limit in.
In practice, most UK first-fix carpenters set floor joists at 400 mm centres by habit and only drop to 600 mm when a designer specifically calls for it. A joiner will often add a row of solid noggins or herringbone strutting at mid-span on anything over about 2.5 m — it's not always on the drawing, but it's what stops a technically-compliant floor from feeling springy underfoot.
A floor that passes on strength can still fail on bounce — the span tables already build the stiffness limit in, so don't second-guess them upwards.
Worked example — a 3.6 m bedroom floor in Leeds
A first-floor bedroom in a Leeds semi, joists spanning 3.6 m clear between the external wall and a load-bearing internal wall, 400 mm centres, C16 carcassing from the local merchant.
- 47 × 175 mm C16 at 400 mm centres spans 3.53 m — just short of 3.6 m.
- 47 × 200 mm C16 at 400 mm centres spans 4.04 m — covers it comfortably.
- So the floor needs 47 × 200 mm C16 at 400 mm centres.
- Alternative: switch to C24 and use 47 × 175 mm (spans 3.81 m in C24) if joist depth is tight against a low ceiling.
Timber cost check: 47 × 200 mm C16 carcassing is widely stocked at Travis Perkins and Jewson; at 2026 merchant prices expect roughly £9–£13 per metre, so a room of eight 3.9 m joists is around £280–£400 in timber before fixings.
When a size lands just short, stepping up one depth is nearly always cheaper and simpler than dropping the spacing across the whole floor.
When to stop using a span table
Span tables cover standard, evenly loaded, uninterrupted domestic floors. Several situations require a structural engineer instead:
- Trimming around a stairwell or loft hatch — trimmer joists carry extra load from cut-off ends and need individual calculation.
- Joists carrying a stud wall or a water tank — concentrated load the domestic table doesn't cover.
- Notched or drilled joists near mid-span — pipes and cables run through floor joists, but cutting them incorrectly reduces stiffness sharply.
- Spans over approximately 5 m — standard solid timber usually gives way to engineered joists (I-joists, posi-joists) or steelwork at this range.
- Any loft conversion — which changes the imposed load and is always a Building Control job.
Merchants like Travis Perkins and Jewson stock C16 as standard carcassing; you usually have to ask for C24 and wait for it, which is worth knowing before you design around it.
Any joist carrying a wall, a tank or a stairwell opening is an engineer's calculation, not a table lookup.
Frequently Asked Questions
Related tools
- Rafter Length Calculator UK — set out the roof once the floor's done
- Timber Calculator — lengths, board feet and cost for the joists you've sized
- Stud Wall Calculator UK — timber and fixings for the walls that carry them
- Decking Area Calculator — joists and boards for an outdoor deck
Guides worth reading
- How Far Apart Should Floor Joists Be? A UK Guide — spacing, span and the joist size you actually need
Disclaimer: This calculator gives indicative maximum spans for standard domestic timber floors (imposed load 1.5 kN/m², dead load 0.25–0.50 kN/m²) based on the abridged Building Regulations Table A1 (NHBC / TRADA) domestic floor joist spans, calculated on finished/regularised sections. It is a planning and budgeting guide only and is not a structural design. It does not cover joists carrying walls, tanks or stairwell trimming, notched or drilled joists, loft conversions, or non-standard loads. All structural work must be confirmed by Building Control, and non-standard cases must be designed by a qualified structural engineer, in line with BS EN 1995-1-1 (Eurocode 5) and BS EN 1991-1-1. Always confirm against the current published span tables and your timber's actual grade before ordering or building.