Floor Joist Span Calculator (C24 & C16): Max Clear Span and Joist Count

Pick the timber grade, joist size and spacing and this floor joist span calculator gives you an indicative maximum clear span for a normal domestic floor, plus how many joists the room needs. Spans are based on a 1.5 kN/m² imposed load. Treat the result as a planning guide, not a structural sign-off.

Imposed load is set at 1.5 kN/m², the standard for a normal domestic floor (bedrooms, living rooms) under UK guidance. Spans are clear spans between supports, before any deflection or notching checks. The joist count assumes joists run across the room width you enter, with one joist at each end.

How it is calculated

This tool does two separate jobs.

1. Maximum clear span. It reads an indicative value from a built-in span table for domestic floor joists at a 1.5 kN/m² imposed load, the standard residential figure used in UK guidance. The table is built around C24 timber, the grade most new joists are supplied in. For C16, a lower strength class, the calculator reduces the C24 span by about 6 percent, which is a typical real-world difference. Closer centres carry more load each, so as spacing widens from 400 mm to 450 mm to 600 mm the safe span gets shorter. There is no clean formula for span: it comes from bending, shear and deflection limits combined, which is why builders use lookup tables rather than a single sum.

2. Number of joists. This part is simple arithmetic:

joists = floor(room width in m ÷ spacing in m) + 1

The "+1" accounts for the joist at the far end of the room. For example a 3.6 m wide room at 400 mm (0.4 m) centres gives floor(3.6 ÷ 0.4) + 1 = floor(9) + 1 = 10 joists. The clear span figures are clear spans between supports and carry no extra wastage factor, because joists are cut to length on site; order each joist slightly long and trim to the exact bearing.

Worked example

Say you are decking out a 3.6 m wide bedroom with C24 joists at 400 mm centres, and you want to use 47 × 150 mm timber.

• From the table, C24 47 × 150 at 400 mm centres spans about 2.90 m.
• Your span is 3.6 m, which is more than 2.90 m, so 47 × 150 will not reach. Step up to 47 × 175 (about 3.39 m), still short, or 47 × 200 (about 3.89 m), which clears 3.6 m comfortably.
• Joist count at 400 mm centres: floor(3.6 ÷ 0.4) + 1 = 10 joists.

So a workable answer is ten lengths of 47 × 200 C24, each cut to suit the bearing. An engineer would then confirm the dead load, bearing and any notching for pipes or cables.

Frequently asked questions

How does a floor joist span calculator for C24 timber work?

A floor joist span calculator for C24 takes the joist size and spacing you choose and looks up the maximum safe clear span from a span table for a domestic floor at 1.5 kN/m² imposed load. C24 is a higher strength class than C16, so for the same size it spans a little further. The calculator on this page covers the common sizes from 47 × 100 up to 47 × 250 at 400, 450 and 600 mm centres.

What is the maximum span for a C24 floor joist?

It depends entirely on the joist depth and the spacing. As a rough guide, at 400 mm centres a C24 joist spans about 1.9 m at 47 × 100, about 2.9 m at 47 × 150, about 3.9 m at 47 × 200 and about 4.4 m at 47 × 225. Wider 600 mm centres reduce those spans by roughly 10 to 15 percent. Always confirm against the current span tables for your exact loading.

Is this the same as a joist span calculator for any timber?

The method is the same general joist span calculator approach, but the numbers here are specific to C16 and C24 structural softwood at a domestic 1.5 kN/m² imposed load. Different grades, glulam, I-joists or higher imposed loads (such as commercial floors) use different tables, so do not apply these figures to those.

How many joists do I need for my room?

Use the timber flooring calculator part of this tool: enter the room width measured across the direction the joists run. The count is floor(width ÷ spacing) + 1. A 4 m room at 400 mm centres needs floor(4 ÷ 0.4) + 1 = 11 joists. Add a trimmer or doubled joists under heavy point loads such as a bath or a stud wall above.

What size joist do I need to span 4 metres?

For a 4 m clear span on a domestic floor in C24 at 400 mm centres you are looking at roughly 47 × 220 to 47 × 250, since 47 × 200 tops out near 3.9 m. Going to 600 mm centres or dropping to C16 pushes you to a deeper section again. This is exactly the kind of borderline case to confirm with an engineer.

Does this timber calculator handle the difference between C16 and C24?

Yes. Pick the grade in the dropdown. C24 uses the base span table; C16 is shown at about 6 percent less span for the same size and spacing, which reflects its lower strength and stiffness. If your merchant only stocks one grade, size the joist for that grade rather than swapping grades at the same depth.

Can I use this as a lintel size calculator?

No. This is a floor joist span calculator, not a lintel size calculator. Lintels over door and window openings carry concentrated loads from the wall and floor above and are usually steel or concrete, sized from manufacturer load tables (for example catnic or IG) by a structural engineer. Do not size a lintel from joist span figures.

Are these spans safe to build from directly?

No. They are indicative planning figures only. Real joist design depends on the actual dead load of your floor build-up, the imposed load, bearing length, notch and hole positions, partitions or tanks above, and the latest span tables (BS 5268, TRADA or Eurocode 5). Always have the floor checked and signed off by a structural engineer or your building control surveyor before ordering timber or building.