Joist and decking calculator
Enter the span, the width of the floor or deck and the centres, and this returns the joist count, the centres you actually achieve, the lineal metres to order and the noggin run. It is a quantity take-off. Whether a given joist can carry the span is a separate question, answered below.
Take-off and span check are two different jobs
Counting joists is arithmetic. Deciding whether a 47 by 195 C16 joist can carry 3.6 metres is a structural question, and it depends on the grade, the section, the centres, the load and the deflection limit. The calculator above does the first one honestly and does not pretend to do the second.
For the span itself, the free source most British builders use is the TRADA span tables published in the Eurocode 5 span tables for solid timber members. Building control will accept a span read from a recognised table for a straightforward domestic floor. Anything unusual, and anything carrying a point load such as a partition or a bath, wants a structural engineer rather than a table.
Centres: 400, 450 or 600
Closer centres let a smaller joist span further and give a stiffer floor. Wider centres use less timber and go up faster. 400mm is the common choice for a domestic floor, 600mm suits a deck with a thicker board, and 450mm sits between them.
The calculator reports the centres you actually achieve as well as the ones you asked for. Those are rarely the same number, because a floor width is not usually a whole multiple of the spacing. The last bay ends up tighter than the rest, and it is better to know that before the boards go down than after.
Noggins are not optional
Strutting between joists stops them twisting under load and makes a noticeable difference to how solid a floor feels. One row at mid span is the usual minimum for a domestic floor. On longer spans, two rows at the third points is better. The calculator returns the lineal metres so it goes on the order rather than being cut from offcuts on the day.
What the take-off leaves out
The figure above is net. It carries nothing for waste, nothing for trimmers around a stair opening or a hearth, and nothing for the doubled joists that go under a partition running parallel to the span. Those are real timber and they belong on the order. Add them from the drawing.
Turn the take-off into a priced estimate
Built Simple takes the quantities straight into an estimate you can send, then keeps the programme and the crew on the same job. Free to start.
Start free with Built SimpleFrequently Asked Questions
How far can a floor joist span? It depends on the section, the grade, the centres and the load, so there is no single answer. Read it from a recognised span table such as the TRADA Eurocode 5 tables, or have an engineer check it. The calculator above handles the quantities rather than the span.
What centres should floor joists be at? 400mm is the usual choice for a domestic floor. 600mm is common on decking with a thicker board. 450mm sits between them. Closer centres allow a smaller section to span further.
Do I need noggins between joists? Yes for any normal domestic floor. One row of strutting at mid span is the usual minimum and two rows at the third points is better on a long span. It stops the joists twisting and it is the difference between a solid floor and a bouncy one.
How many joists do I need? Divide the floor width by the centres and add one, which is what the calculator does. Then add trimmers around any opening and doubled joists under a parallel partition, which it does not.
Does a deck need building regulations approval? A ground level deck usually does not. Once it is raised, guarding and structure come into play, and a deck attached to the house or over a metre high needs checking against Approved Documents A and K.
Related calculators and guides
Sources
- Eurocode 5 span tables for solid timber members in floors, ceilings, roofs and wall plates, TRADA. Paid publication, referenced rather than reproduced.
- Approved Document A, Structure and Approved Document K, HM Government.
