Rebar Calculator
By Taro Schenker | Updated February 11, 2026
Quick Answer: To calculate rebar for a slab, divide each dimension (ft) by the spacing (ft) and add 1 to get bars per direction. Total linear feet = (bars lengthwise x length) + (bars widthwise x width), plus splice overlaps. Weight = linear feet x weight per foot for your bar size. Common weights: #3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502 lbs/ft. Add 10% for waste. For a 20 x 20 ft slab at 12-inch spacing, you need 42 bars (21 each way), about 840 linear feet of #4 rebar.
Calculate how much rebar you need for concrete slabs, walls, footings, and columns. Enter your dimensions, bar size, and spacing to get the total number of bars, linear footage, weight, and estimated cost.
How to Calculate Rebar
The formula for calculating rebar in a concrete slab (grid pattern):
Total Linear Ft = (Bars lengthwise × Length) + (Bars widthwise × Width)
- Measure the length and width (or height) of your concrete structure in feet.
- Choose your rebar size based on the structural requirements (#4 is most common for residential work).
- Divide each dimension by the spacing to find how many bars run in each direction, then add one.
- Multiply the bar count by the run length to get total linear feet per direction.
- Add splice overlap lengths where bars need to join.
- Add 10% for waste from cuts, bends, and layout adjustments.
Rebar Size Reference
Common rebar sizes used in residential and commercial construction:
| Size | Diameter | Weight/ft | Typical Use |
|---|---|---|---|
| #3 | 3/8" | 0.376 lbs | Walkways, light slabs |
| #4 | 1/2" | 0.668 lbs | Driveways, patios, residential slabs |
| #5 | 5/8" | 1.043 lbs | Footings, retaining walls |
| #6 | 3/4" | 1.502 lbs | Structural walls, commercial work |
| #7 | 7/8" | 2.044 lbs | Heavy structural, columns |
| #8 | 1" | 2.670 lbs | Heavy commercial, bridge decks |
Rebar Spacing Guide
- 6 inches: Heavy structural applications, load-bearing walls, and foundations in seismic zones.
- 8 inches: Structural footings, basement walls, and retaining walls over 4 feet tall.
- 12 inches: Standard residential slabs, driveways, garage floors, and patios. The most common spacing for general use.
- 16 inches: Light-duty slabs, sidewalks, and non-structural flatwork where code allows wider spacing.
- 18 inches: Minimum reinforcement for slabs on grade with low load requirements.
Formula Summary
Rebar Grid Formula (Slab): To calculate rebar for a slab, determine how many bars run in each direction based on the spacing, then compute total linear footage including splice overlaps.
$$N_{\text{lengthwise}} = \lfloor \frac{W}{S} \rfloor + 1 \quad,\quad N_{\text{widthwise}} = \lfloor \frac{L}{S} \rfloor + 1$$
$$LF_{\text{total}} = N_{\text{lengthwise}} \times L + N_{\text{widthwise}} \times W + \text{splices} \times O$$
Adjusted Linear Feet: $$LF_{\text{adj}} = LF_{\text{total}} \times (1 + W_f)$$ where $W_f$ is the waste factor (default 10%).
Weight Formula: $$W_{\text{lbs}} = LF_{\text{adj}} \times \rho$$ where $\rho$ is the weight per linear foot for the chosen bar size.
| Rebar Size | Diameter (in) | Weight (lbs/ft) |
|---|---|---|
| #3 | 3/8 | 0.376 |
| #4 | 1/2 | 0.668 |
| #5 | 5/8 | 1.043 |
| #6 | 3/4 | 1.502 |
| #7 | 7/8 | 2.044 |
| #8 | 1 | 2.670 |
Standard Assumptions: Waste factor of 10% accounts for cuts, bends, and layout adjustments. Splice overlap = 40 x bar diameter. Standard bar lengths are 20 ft. Spacing is center-to-center in inches, converted to feet for calculation.
Related Calculators
- Concrete Calculator — calculate concrete volume for slabs, footings, and columns
- Concrete Block Calculator — estimate blocks and mortar for walls
- Brick Calculator — calculate bricks for walls, patios, and paths
Estimates are for planning purposes only. Consult a qualified contractor for critical projects. Actual material requirements may vary based on site conditions, waste, and installation methods.
Pricing last verified: February 2026. Material prices vary by region, season, and supplier. Prices shown are national averages for planning purposes.