How it works
A grid of rebar runs bars in both directions. For each direction, the number of bars is the usable dimension (the dimension less cover at both edges) divided by the spacing, rounded down, plus one. Each bar is the other dimension less cover.
Total length is the sum of all the bars, plus an allowance for laps, hooks, and waste. Weight uses the nominal weight per foot from ASTM A615. Bars come in 20 ft sticks, so divide the total by 20 and round up.
Worked example
A 20 ft × 10 ft slab with #4 bars at 16 in on center, 3 in cover, and 10% allowance:
- Bars along the length = (120 − 2 × 3) ÷ 16 + 1 = 8, each 19.5 ft
- Bars along the width = (240 − 2 × 3) ÷ 16 + 1 = 15, each 9.5 ft
- Total length with 10% = 328 ft
- Weight = 328 ft × 0.668 lb/ft = 219 lb
| Input | Value |
|---|---|
| Slab length | 20 ft |
| Slab width | 10 ft |
| Bar spacing (on center) | 16 in |
| Cover at edges | 3 in |
| Bar size | #4 (1/2 in) |
| Allowance for laps and waste | 10 % |
| Result | Value |
|---|---|
| Total weight with allowance | 219 lb |
| Total bar length with allowance | 328 ft |
| Bars running along the length | 8 bars |
| Bars running along the width | 15 bars |
| 20 ft sticks to buy | 17 sticks |
Nominal weight per foot (ASTM A615)
| Bar size | lb per foot |
|---|---|
| #3 (3/8 in) | 0.376 |
| #4 (1/2 in) | 0.668 |
| #5 (5/8 in) | 1.043 |
| #6 (3/4 in) | 1.502 |
| #7 (7/8 in) | 2.044 |
| #8 (1 in) | 2.67 |
Show the nominal weight per foot (astm a615) table
| Bar size | lb per foot |
|---|---|
| #3 (3/8 in) | 0.376 |
| #4 (1/2 in) | 0.668 |
| #5 (5/8 in) | 1.043 |
| #6 (3/4 in) | 1.502 |
| #7 (7/8 in) | 2.044 |
| #8 (1 in) | 2.67 |
ASTM allows about ±3.5% on weight.
Assumptions and limits
- One layer of bars in each direction at the same spacing. Two-layer mats, chairs, and dowels are not counted.
- Rebar size, spacing, and cover come from the plans or your engineer; this tool does not choose them.
- Lap splices and hooks add length; the allowance is a rough placeholder that you set.
Common questions
How many rebar do I need for a slab?
Count the bars in each direction from the spacing and cover, multiply by each bar’s length, and add an allowance. The calculator does that.
How much does #4 rebar weigh?
0.668 lb per foot, so a 20 ft stick weighs about 13.4 lb.
What spacing should I use?
Use the plans, your engineer, or the local code; this tool takes it as input.
Does the weight include laps?
Only through the allowance you set. Lap lengths depend on bar size and concrete strength.
Sources
- ASTM A615 standard bar sizes and nominal weights per foot (#3 0.376, #4 0.668, #5 1.043, #6 1.502, #7 2.044, #8 2.670 lb/ft).
- Grid counting: bars = usable width ÷ spacing + 1 in each direction.
Updated 2026-09-30