Rebar Calculator
Work out how many feet of rebar, how many 20 foot bars and how many pounds you need for a slab, a footing or a wall. Pick the bar size, set the spacing and add lap and waste.
Results are estimates based on standard ASTM bar weights. Laps, hooks, dowels and accessories vary by project, so check quantities against your structural drawings before ordering.
How to Calculate Rebar
For a slab with a rebar grid, count the bars in each direction. Take the slab width, subtract the edge cover on both sides, divide by the spacing and add one. Each of those bars runs the length of the slab minus cover. Then do the same in the other direction.
Add up the linear feet in both directions, add an allowance for laps and waste, and multiply by the weight per foot for your bar size to get pounds. Divide by 2,000 for tons.
Example
A 20 by 30 foot slab with #4 bars at 12 inches each way and 3 inches of cover needs 30 bars that are 19.5 feet long and 20 bars that are 29.5 feet long. That is 1,175 linear feet, or about 1,293 feet with 10% for laps and waste. At 0.668 lb per foot that is roughly 863 lb of rebar.
| Bar size | Diameter | Weight per foot |
|---|---|---|
| #3 | 0.375 inch | 0.376 lb |
| #4 | 0.500 inch | 0.668 lb |
| #5 | 0.625 inch | 1.043 lb |
| #6 | 0.750 inch | 1.502 lb |
| #7 | 0.875 inch | 2.044 lb |
| #8 | 1.000 inch | 2.670 lb |
Tips for an accurate rebar count
- Check the structural notes for lap lengths. Laps are often 40 or more bar diameters and add up fast on long runs.
- Count dowels, corner bars, hooks and stirrups as separate items.
- Add chairs, tie wire and other accessories to your material list.
- Use the bar size and spacing shown on the drawings, not a rule of thumb.
Rebar Calculator Questions
How much does #4 rebar weigh per foot?
A #4 bar weighs about 0.668 lb per foot, so a 20 foot bar weighs about 13.4 lb.
What spacing is common for slab rebar?
Many slabs use #3 or #4 bars at 12 to 18 inches each way, but always follow the spacing on your structural drawings.
How much should I add for laps and waste?
Many estimators start with about 10% for laps and waste on slabs, and more on walls and footings with many splices.
Does this calculator include concrete?
No. Use our concrete calculator for cubic yards, or send your plans for a full concrete and rebar takeoff.
Can you do the full rebar takeoff for my bid?
Yes. We take off every bar, lap, dowel and accessory from your structural drawings and deliver weights by bar size in editable Excel, usually within 24 to 48 hours.
Need a Full Rebar Takeoff?
Send your structural drawings and get rebar by bar size, length and weight in editable Excel, usually within 24 to 48 hours.
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