Find the right joist spacing and timber size for your span
Getting the right joist spacing is critical for a safe, solid floor that meets UK Building Regulations. Floor joists must be strong enough to carry the imposed loads without excessive deflection (bounce), and the spacing between them determines how much load each joist carries. In UK domestic construction, floor joists are typically spaced at either 400mm or 600mm centres, with the choice depending on the span, timber size, and the type of load the floor must support. This calculator uses simplified span tables derived from BS 8103 (Structural Design of Low-rise Buildings) and Approved Document A of the Building Regulations. It covers standard C16 graded softwood timber in common UK sizes from 47x100mm to 47x225mm. Whether you are building a new floor, replacing joists in a renovation, or checking whether existing joists are adequate for a new use, this tool helps you quickly determine the right combination of timber size and spacing for your specific span.
To determine the correct joist spacing: 1. Measure and enter the clear span in metres. This is the unsupported distance between the inner faces of the supporting walls or beams. For most domestic rooms, this is the room width. If a joist has intermediate support (such as a sleeper wall or steel beam), use the longest unsupported section. 2. Select the load type. "Domestic Floor" is the standard choice for habitable rooms, hallways, and landings, based on an imposed load of 1.5 kN/m2 plus dead loads. "Ceiling Only" applies to joists that support only a plasterboard ceiling with light storage in the loft, allowing approximately 10% longer spans. "Flat Roof" applies to joists supporting a flat roof covering, which require approximately 10% shorter spans due to the different loading pattern and the need to resist wind uplift. 3. Select your timber size. Standard UK structural softwood comes in regularised sizes: 47mm wide by depths of 100, 125, 150, 175, 200, or 225mm. The depth is the critical dimension for bending resistance. If in doubt, 47x200mm is the most common choice for domestic floor joists in the UK. 4. Review the results. The calculator shows whether your selected timber is suitable for the span at 400mm or 600mm centres, the maximum span achievable at each spacing, and how many joists you would need for a typical 4m room width. If the timber is too small, the calculator recommends the next size up.
The joist spacing calculation is based on lookup tables rather than formulas, reflecting how joist design works in practice. UK Building Regulations Approved Document A and BS 8103-3 provide span tables that have been calculated by structural engineers considering bending stress, shear stress, deflection limits, and vibration criteria. The span tables used in this calculator are for C16 graded softwood (the standard UK structural timber grade) with the following assumptions: imposed load of 1.5 kN/m2 for domestic floors (as per BS EN 1991-1-1 UK National Annex), dead load of approximately 0.5 kN/m2 (plasterboard ceiling, timber boarding, and light finishes), and a maximum deflection limit of span/300 or 14mm, whichever is less. For example, a 47x200mm C16 joist can span up to 3.63m at 400mm centres or 3.30m at 600mm centres under domestic floor loading. At 600mm centres, each joist carries 50% more load per joist (because the tributary width is 600mm instead of 400mm), which is why the maximum span is shorter. The load type adjustments work as follows: ceiling joists carry much less load (typically 0.25 kN/m2 imposed for maintenance access only), so they can span approximately 10% further. Flat roof joists must resist wind suction as well as imposed loads, and the deflection limits are stricter to maintain falls for drainage, reducing the allowable span by approximately 10%. When the calculator shows your timber is too small, it searches the span table for the next size that can accommodate your span at 400mm centres, providing a clear upgrade path.
These span tables are simplified and based on C16 timber. For C24 grade timber (which is stronger but more expensive), spans can be approximately 10-15% longer. Always verify your joist specification with Building Control before construction begins. In renovation projects, existing joists may be imperial sizes (e.g. 2x8 inches) that do not match modern metric sizes exactly. If you are adding additional load to an existing floor (such as replacing carpet with stone tiles, or converting a loft), the existing joists may need to be supplemented or replaced. For spans exceeding the tables in this calculator, engineered timber joists (such as I-joists or metal web joists) offer longer spans with shallower depths and are increasingly common in UK new-build construction.