What is Rebar?
Rebar (reinforcement steel / reinforcing steel) is a building material used to improve the properties of concrete blocks and slabs. These steel bars are made of patterned (deformed) steel to improve adhesion to concrete, and they are typically arranged in a mesh grid pattern. Thanks to steel's thermal expansion coefficient being very similar to concrete's, reinforcement compensates for the low tensile strength of concrete. Rebar can boost resistance to concrete failure by several times.
Civil engineers apply reinforcing steel when designing buildings, poured concrete driveways, foundations, and slabs. Besides increased tension resistance, rebar also improves concrete cracking resilience and allows for reducing the thickness of concrete blocks. Applying reinforcement is more expensive upfront, but reinforced concrete structures last significantly longer — making it a money-saving decision in the long run.
Rebar Sizes
For imperial-unit countries, bar sizes give the diameter in units of ⅛ inch for bar sizes #2 through #8 (e.g., #4 = 4/8 = ½ inch diameter). For metric countries, bar sizes indicate the nominal diameter in millimeters.
Imperial vs. Metric Rebar Size Table
| Imperial Bar | Metric Bar (soft) | Diameter (in) | Diameter (mm) | Weight (lb/ft) | Weight (kg/m) |
|---|---|---|---|---|---|
| #2 | No.6 | 0.250 | 6.35 | 0.167 | 0.249 |
| #3 | No.10 | 0.375 | 9.525 | 0.376 | 0.560 |
| #4 | No.13 | 0.500 | 12.7 | 0.668 | 0.994 |
| #5 | No.16 | 0.625 | 15.875 | 1.043 | 1.552 |
| #6 | No.19 | 0.750 | 19.05 | 1.502 | 2.235 |
| #7 | No.22 | 0.875 | 22.225 | 2.044 | 3.042 |
| #8 | No.25 | 1.000 | 25.4 | 2.670 | 3.973 |
| #9 | No.29 | 1.128 | 28.65 | 3.400 | 5.060 |
| #10 | No.32 | 1.270 | 32.26 | 4.303 | 6.404 |
| #11 | No.36 | 1.410 | 35.81 | 5.313 | 7.907 |
| #14 | No.43 | 1.693 | 43.0 | 7.650 | 11.390 |
| #18 | No.57 | 2.257 | 57.3 | 13.600 | 20.240 |
How to Use the Rebar Calculator
- Choose your measurement system — Imperial (feet & inches) or Metric (metres & millimetres).
- Enter slab dimensions — the length and width of your concrete slab.
- Select bar size — choose from standard imperial bar sizes (#2–#18) or metric diameters (6–40 mm).
- Enter bar spacing — the center-to-center distance between adjacent bars. Common values are 6 in (150 mm) for residential slabs or 12 in (300 mm) for lightly loaded areas.
- Enter price per unit (optional) — the cost per linear foot or metre of rebar. Leave at 0 if you only need quantity estimates.
- Select currency — the calculator supports 20 world currencies including USD, RUB, EUR, GBP, and more.
- Click Calculate — see the number of bars, total rebar length, weight (in both lb and kg), and total cost.
Rebar Cost — An Example Calculation
Suppose you need to reinforce a 20 ft × 20 ft concrete patio slab with #4 rebar at 6-inch spacing:
- Bars along the length direction: floor(20 / 0.5) + 1 = 41 bars × 20 ft = 820 ft
- Bars along the width direction: floor(20 / 0.5) + 1 = 41 bars × 20 ft = 820 ft
- Total rebar length: 1,640 ft
- Total weight: 1,640 × 0.668 lb/ft ≈ 1,095 lb (≈ 497 kg)
- Cost at $0.85/ft: 1,640 × $0.85 ≈ $1,394
FAQs
What bar size should I use for a residential concrete slab?
#4 rebar (½ inch diameter) is the most common choice for residential slabs, driveways, and patios. For heavier loads — such as garage floors or industrial slabs — #5 or #6 bars are recommended.
What is the standard rebar spacing for a concrete slab?
The most common spacing is 12 inches (300 mm) on center for lightly loaded residential slabs and 6 inches (150 mm) for more heavily loaded floors or areas with poor soil conditions. Always consult local building codes.
How much rebar do I need per square foot?
It depends on bar size and spacing. At 12-inch spacing with #4 rebar, you need approximately 2 bars per linear foot in each direction. The calculator determines the exact count and total length for your specific dimensions.
Does rebar rust inside concrete?
Standard carbon steel rebar can eventually corrode if moisture penetrates the concrete. For aggressive environments, consider epoxy-coated, galvanized, or stainless-steel rebar, which cost more but offer superior corrosion resistance.
How do I calculate rebar weight?
Multiply the total rebar length by the weight per unit length for the bar size chosen. For example, #4 bar weighs 0.668 lb/ft; 100 ft of #4 rebar weighs 66.8 lb. The calculator handles this automatically for all supported bar sizes.