Shed Floor Joist Count
Joists run across the width at a fixed spacing, each one spanning the floor's length. Dividing the width by the spacing, rounding up, and adding one for the starting joist gives the count; multiplying by the length gives the total lumber to buy.
Your numbers
Joists
9
Total lumber length
36m
Formula
Joists = ⌈Floor width (across joists) × 100 ÷ Joist spacing (cm)⌉ + 1, Total lumber length = Joists × Floor length (joist run)
Matching the spacing to your floor-sheet size (e.g. plywood dimensions) keeps seams landing on a joist, which cuts down on sag.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Floor width (across joists) (m) | 3 | 0.5 and up |
| Floor length (joist run) (m) | 4 | 0.5 and up |
| Joist spacing (cm) (cm) | 40 | 10 and up |
Step by step
Quick reference table
Results when only Floor width (across joists) (m) changes and everything else stays put.
| Floor width (across joists) (m) | Joists | Total lumber length (m) |
|---|---|---|
| 1.5 | 5 | 20 |
| 2.25 | 7 | 28 |
| 3 | 9 | 36 |
| 4.5 | 13 | 52 |
| 6 | 16 | 64 |
What each result means
| Result | At default values |
|---|---|
| JoistsFloor width divided by joist spacing, rounded up, plus one. | 9 |
| Total lumber length (m)Rafter count times the length of one — the total lumber to order. | 36 |
Common mistakes
Matching the spacing to your floor-sheet size (e.g. plywood dimensions) keeps seams landing on a joist, which cuts down on sag.
Glossary
- Joists
- Floor width divided by joist spacing, rounded up, plus one.
- Total lumber length
- Rafter count times the length of one — the total lumber to order.
Frequently asked questions
QHow is Shed Floor Joist Count calculated?
Joists = ⌈Floor width (across joists) × 100 ÷ Joist spacing (cm)⌉ + 1, Total lumber length = Joists × Floor length (joist run) — Joists run across the width at a fixed spacing, each one spanning the floor's length. Dividing the width by the spacing, rounding up, and adding one for the starting joist gives the count; multiplying by the length gives the total lumber to buy.
QCan you walk through an example?
With Floor width (across joists) 3m, Floor length (joist run) 4m, Joist spacing (cm) 40cm, the answer is Joists 9.
QWhat do I need to enter?
Enter Floor width (across joists), Floor length (joist run), Joist spacing (cm). The result recalculates as you type, and an empty box counts as zero.
QHow much does the answer move if I change a number?
Changing only Floor width (across joists) (m) moves the answer to Floor width (across joists) (m) 1.5 → Joists 5 and Floor width (across joists) (m) 6 → Joists 16. The table below lays out five steps.
QHow are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
QAnything to watch out for?
Matching the spacing to your floor-sheet size (e.g. plywood dimensions) keeps seams landing on a joist, which cuts down on sag.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.