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Drywall, Flooring & Interior

Floor Joist Calculator

Joist count, lengths and blocking for a floor, from the span and the spacing your design calls for.

Floor layout

This counts material for a spacing you specify. It does not check whether the span is safe — see the note below.

The floor
Timber
Used for the timber volume only. Whether this size suits your span comes from a span table or an engineer.

Your result

Sample values

16

Joists

14

Joist length, ft

236

Linear feet of joist

1

Rows of blocking

20 ft of floor, 14 ft span, 16 in centres, 5% waste.

What to order

Item Quantity
Joists, 14 ft lengths 16
Linear feet, including waste 236 ft
Blocking rows 1
Rim board 40 ft

How to use this calculator

Enter the length of floor the joists repeat along, and the span each joist crosses between supports. Those two are easy to mix up: the span is how far a single joist reaches, and the length is the direction you march along placing them.

Pick the spacing your design calls for. Sixteen inch centres is the residential default, twelve is used under tile and heavy loads, and twenty-four generally means engineered I-joists rather than sawn timber.

Formula

Joists = floor(Floor length ÷ Spacing) + 1

Linear ft = Joists × Span × (1 + Waste)

Blocking = ceil(Span ÷ 8) − 1 rows

+ 1
a joist at both ends — ten spaces need eleven joists
Blocking
a row at roughly 8 ft intervals to stop the joists twisting

Rim board is estimated as twice the floor length. Doubled joists under partitions are not included.

Worked example

A floor 20 ft long with a 14 ft span at 16 in centres, 5% waste.

  1. Floor length: 20 ft = 240 in
  2. Joists: floor(240 ÷ 16) = 15 spaces, + 1 end joist = 16
  3. Linear feet: 16 × 14 = 224 ft, × 1.05 = 236 ft
  4. Blocking: ceil(14 ÷ 8) − 1 = 1 row

16 joists at 14 ft, one row of blocking

Understanding your results

This is a material count, not a structural design. It tells you how many joists fit at the spacing you choose and how much timber that is. It does not tell you whether a 2 × 10 will carry your floor over that span, because that depends on species, grade, load and deflection limits.

Use a published span table for your timber species and grade, or have an engineer size the floor, and then come back here for the quantities. Anything unusual — a point load, a long span, a floor carrying a bathroom or a partition — needs that check rather than a rule of thumb.

Questions

About this calculator

Ask something else

No, and deliberately so. Joist sizing depends on species, grade, live and dead loads, and the deflection limit you are designing to. Use a span table for your timber or have it engineered, then use this to count material at that size.

The span is how far one joist reaches between supports. The floor length is the direction you repeat along placing them. A 20 ft by 14 ft room joisted the short way has a 14 ft span and a 20 ft length, and swapping them gives a very different count.

Because there is a joist at each end. Fifteen spaces need sixteen joists, plus the end one makes seventeen. It is the same off-by-one that catches people on fence posts and roof trusses.

Most codes ask for a row where the span exceeds about eight feet, to stop deep joists rolling under load. This estimates rows at roughly eight foot intervals, but the requirement comes from your code and joist depth, not from this figure.

Keep going

All Drywall, Flooring & Interior tools
Estimates, not specifications. Results are based on the values you enter and on typical material properties. Densities, coverage rates and product sizes vary between suppliers, so confirm quantities against your supplier's data before ordering. Structural work must be signed off by a qualified professional and meet your local building code.