Rebar Calculator
Use this rebar calculator for rectangular slab grids. Enter slab size, spacing, edge clearance, layers, stock length, lap allowance, waste, and optional price per stick.
Results
This calculator estimates rectangular slab rebar takeoff only. It does not design reinforcement, check code compliance, calculate development length, or replace project plans.
How to use this calculator
- 01Enter the slab length and width.
- 02Enter edge clearance, then the bar spacing in each direction.
- 03Choose rebar size, one mat or two mats, and stock bar length.
- 04Add lap allowance and waste percentage.
- 05Open the cost section if you want material cost from price per stick.
- 06Read bar counts, total rebar length, sticks to buy, weight, and optional cost.
Understanding the math
The calculator subtracts edge clearance from the slab, counts bars in both directions, then multiplies by bar length. Lap and waste are added before stock sticks are rounded up.
usable length = slab length - 2 x edge clearance usable width = slab width - 2 x edge clearance bar count = floor(usable dimension / spacing) + 1 raw length = (lengthwise bars x usable length + widthwise bars x usable width) x layers order length = raw length x (1 + lap %) x (1 + waste %)
Example: a 20 ft x 12 ft slab with 3 in edge clearance and 18 in spacing has a 19.5 ft x 11.5 ft usable grid. One mat uses 8 lengthwise bars and 14 widthwise bars, or 317 ft of raw rebar. With 10% waste, order about 349 ft, or 18 sticks at 20 ft each.
Need concrete volume for the same slab? Use the Concrete Calculator.
Rebar quick reference
Use these values as a takeoff check. If your drawings list a different bar size, spacing, or cover, use the drawing values.
| Item | Value | Notes |
|---|---|---|
| #3 rebar weight | 0.376 lb/ft | Light slab and tie work |
| #4 rebar weight | 0.668 lb/ft | Common residential slab size |
| #5 rebar weight | 1.043 lb/ft | Heavier slab and footing work |
| #6 rebar weight | 1.502 lb/ft | Use when shown on plans |
| #8 rebar weight | 2.670 lb/ft | Larger bars need plan details |
| Common slab spacing | 12 in to 24 in | Follow the project detail |
| Typical edge clearance | About 2 in to 3 in | Use the plan or local spec |
| Count formula | floor(usable width / spacing) + 1 | Includes both edge bars |
Frequently asked questions
How to calculate rebar?
Start with the slab size, subtract edge clearance, then count bars in both directions based on spacing. Multiply each bar count by its run length to get total rebar length.
How much rebar do I need for a slab?
It depends on slab length, slab width, bar spacing, number of mats, lap allowance, and waste. The calculator totals those into stock sticks to buy.
How to calculate rebar for concrete slab?
Count lengthwise bars across the usable slab width and widthwise bars across the usable slab length, then multiply each group by its bar length.
What rebar spacing should I use?
Spacing depends on the slab design and local code or specs. Common takeoff spacing is often 12 in to 24 in, but follow the project plans or engineer's detail.
Can I use this calculator with metric measurements?
Yes. Use the Imperial / Metric toggle. Inputs, results, examples, and the reference table follow the active unit system where applicable.
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