What this is for, and what it is not
This answers the ordering questions. How many joists, how many metres of timber, what one of them weighs on the way up the stairs, and roughly what depth a span is going to want. Those are the things you need a week before anybody is on site, when you are pricing a job or deciding whether a floor is a morning or a fortnight.
It is not a specification and it does not try to be. It gives you the span a section reaches rather than the section to use, the number it leads with is the count rather than the size, and building control will want a calculation or a table reference from somebody who can sign it. What this saves you is the week of not knowing roughly what you are dealing with.
How to work out how far a joist will span
A domestic floor joist is governed by deflection, not by strength, and that one fact is what makes this calculable rather than a matter of judgement. A joist that is strong enough to never break can still bounce enough to crack a ceiling and rattle the glasses in a cabinet, and that is the limit that bites first on every normal domestic span.
For a simply supported joist carrying an evenly spread load, deflection is 5wL⁴ / 384EI, where I is bd³/12. Set the deflection to the serviceability limit, which for a domestic floor is span over 333 capped at 14mm, and solve for the span. That is the whole calculation, and it is the same one the published tables were built from, which is why the figures here land on them: 47 × 195 in C24 at 400 centres comes out at 4.52m against a printed 4.52.
Depth is worth far more than anything else you can change
Because I goes as the depth cubed. Going from 195 to 220 is a 13% deeper joist and a 43% stiffer one. Doubling the breadth, by bolting two together, only doubles it. That is why the answer to a span that does not quite reach is nearly always a deeper section rather than a wider one or a better grade, and why the grade upgrade everybody reaches for first is the weakest of the three levers.
The centres are the other lever, and they cut both ways
Bringing joists in from 600 to 400 centres gives each one a third less floor to carry, which is worth about 14% more span. It also means half again as many joists, so the timber bill goes up by a third while the span goes up by a seventh. The tool shows both numbers at once because the trade-off is the decision and neither figure makes sense alone.
What it cannot see
Notches and holes take stiffness out and none of this knows where yours are. Point loads break the assumption entirely: a stud partition running the wrong way, or a bath, sits on one or two joists rather than spreading across all of them. Trimmers around a stairwell or a chimney carry other joists and are a different calculation. Bearing is not considered at all, because deflection does not care how the joist lands and the wall very much does. And engineered joists follow their own maker's tables, not these.
Common questions
How far can a 47 x 195 joist span?
About 4.05m in C16 and 4.52m in C24, at 400 centres, carrying a domestic floor. Go out to 600 centres and both drop by around an eighth, because each joist is picking up half again as much floor. Those figures are the point at which the joist reaches its deflection limit, which is what governs a domestic floor rather than strength. Change the grade, the spacing or what the floor is for and the whole table above moves.
Is C24 worth it over C16?
It buys about 11% more span, not the 50% the grade numbers suggest, and that is the single most misunderstood figure in domestic timber. The 50% is BENDING strength, 24 against 16, and bending is not what decides a domestic floor. Stiffness is, and that goes 8000 to 11000, which is 37.5%. Span goes as the cube root of stiffness, so 1.375 becomes 1.112. Eleven per cent is sometimes exactly the eleven per cent you need and it is rarely worth a special order on its own.
How many joists do I need?
Divide the floor width by the centres and ADD ONE, because there is a joist at each end. A 4.2m floor at 400 centres is ten gaps and eleven joists, and counting ten is the commonest off-by-one on a materials list. The tool does it, and it turns the count into total metres and a weight so you can price it and know what you are carrying up the stairs.
Can I use this instead of getting a structural engineer?
No, and it is not trying to be. This is for COUNTING and COSTING: how many joists, how many metres, roughly what depth the span wants, so you can price a job or know what to ask for before anybody is on site. It assumes a plain simply supported joist with the load spread evenly, and it knows nothing about notches, holes, a trimmer round a stairwell, a stud partition landing on one joist, or a bath. Building control will want a calculation or a table reference from somebody who can sign it, and this is neither.
Why is a loft floor different from a room?
It carries a third of the load. Storage and access is 0.5 kN/m2 imposed against 1.5 for a room, so the same timber spans a good deal further, which is why ceiling joists that happily carry boxes are usually nowhere near enough once the loft becomes a bedroom. Converting a loft is the case where people assume they are boarding over and discover they are replacing the floor.
Does this work for I-joists or metal web joists?
No. An engineered joist has its strength in the flanges and spans much further than solid timber of the same depth, and a metal web joist is a different thing again. Both come with the manufacturer's own span tables and those are the only ones that apply. Nothing on this page does. The same warning sits on the notching tool, for the same reason: engineered joists follow different rules and treating them as deep softwood is the mistake that does real damage.