Rebar Grid Calculator
A two-way grid needs bars running each direction. The count for one direction is the other side's length over the spacing, plus one; multiplying by each bar's length gives the total length, which is then divided by the standard stock length (often 6 m) to get how many bars to buy.
Your numbers
Bars needed
35
Total lumber length
209m
Formula
Total lumber length = (Slab width÷(Bar spacing÷100)+1)×Slab length + (Slab length÷(Bar spacing÷100)+1)×Slab width, Bars needed = Total lumber length÷Stock bar length
Lap splices are not included here. In practice, buy 5–10% extra to cover the overlap where bars are tied together.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Slab width (m) | 4 | 0.3 and up |
| Slab length (m) | 5 | 0.3 and up |
| Bar spacing (cm) | 20 | 5 ~ 50 |
| Stock bar length (m) | 6 | 1 ~ 12 |
Step by step
Quick reference table
Results when only Slab width (m) changes and everything else stays put.
| Slab width (m) | Bars needed | Total lumber length (m) |
|---|---|---|
| 2 | 18 | 107 |
| 3 | 27 | 158 |
| 4 | 35 | 209 |
| 6 | 52 | 311 |
| 8 | 69 | 413 |
What each result means
| Result | At default values |
|---|---|
| Bars neededTotal grid length divided by the standard stock bar length (often 6 m). | 35 |
| Total lumber length (m)Rafter count times the length of one — the total lumber to order. | 209 |
Common mistakes
Lap splices are not included here. In practice, buy 5–10% extra to cover the overlap where bars are tied together.
Glossary
- Bars needed
- Total grid length divided by the standard stock bar length (often 6 m).
- Total lumber length
- Rafter count times the length of one — the total lumber to order.
Frequently asked questions
QHow is Rebar Grid Calculator calculated?
Total lumber length = (Slab width÷(Bar spacing÷100)+1)×Slab length + (Slab length÷(Bar spacing÷100)+1)×Slab width, Bars needed = Total lumber length÷Stock bar length — A two-way grid needs bars running each direction. The count for one direction is the other side's length over the spacing, plus one; multiplying by each bar's length gives the total length, which is then divided by the standard stock length (often 6 m) to get how many bars to buy.
QCan you walk through an example?
With Slab width 4m, Slab length 5m, Bar spacing 20cm, Stock bar length 6m, the answer is Bars needed 35.
QWhat do I need to enter?
Enter Slab width, Slab length, Bar spacing, Stock bar length. 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 Slab width (m) moves the answer to Slab width (m) 2 → Bars needed 18 and Slab width (m) 8 → Bars needed 69. 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?
Lap splices are not included here. In practice, buy 5–10% extra to cover the overlap where bars are tied together.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.