Rebar Calculator
Estimate rebar sticks and weight for a two-way slab grid from spacing and bar size.
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What this tool does
Estimate rebar sticks and weight for a two-way slab grid from spacing and bar size. Bar weights are published ASTM A615 nominal weights. Lap splices (typically 40–60 bar diameters) and edge clearance are not added — increase the stick count for long runs that need laps.
How to use the Rebar Calculator
- Enter or select length in ft.
- Enter or select width in ft.
- Enter or select grid spacing in in.
- Enter or select bar size (astm).
- Enter or select stock stick length in ft.
- Read the calculated result; change any measurement to compare alternatives.
Formula
bars one way = ceil(other dimension × 12 / spacing) + 1; total length = bars × bar length summed both ways; sticks = ceil(total / stick length); weight = total × lb per ft
- length
- Length (ft, before metric conversion)
- width
- Width (ft, before metric conversion)
- spacing
- Grid spacing (in, before metric conversion)
- bar
- Bar size (ASTM)
- stick
- Stock stick length (ft, before metric conversion)
Bar weights are published ASTM A615 nominal weights. Lap splices (typically 40–60 bar diameters) and edge clearance are not added — increase the stick count for long runs that need laps.
Worked example
For rebar calculator, the following measurements illustrate the exact method: Length: 20; Width: 10; Grid spacing: 12; Bar size (ASTM): 3; Stock stick length: 20.
Inputs
- Length20 ft
- Width10 ft
- Grid spacing12 in
- Bar size (ASTM)#3 (⅜ in)
- Stock stick length20 ft
Result
- Total rebar length430 ft
- Sticks to buy22
- Bars each way11 × 21
- Rebar weight161.68 lb
Rebar weights (ASTM A615)
| Bar | Diameter (in) | Weight (lb/ft) |
|---|---|---|
| #3 | 0.375 | 0.376 |
| #4 | 0.5 | 0.668 |
| #5 | 0.625 | 1.043 |
| #6 | 0.75 | 1.502 |
| #7 | 0.875 | 2.044 |
| #8 | 1 | 2.67 |
Results explained
- Total rebar length
- Total rebar length from the formula above, expressed in ft before metric conversion. Bar weights are published ASTM A615 nominal weights. Lap splices (typically 40–60 bar diameters) and edge clearance are not added — increase the stick count for long runs that need laps.
- Sticks to buy
- Sticks to buy from the formula above. Bar weights are published ASTM A615 nominal weights. Lap splices (typically 40–60 bar diameters) and edge clearance are not added — increase the stick count for long runs that need laps.
- Bars each way
- Bars each way from the formula above. Bar weights are published ASTM A615 nominal weights. Lap splices (typically 40–60 bar diameters) and edge clearance are not added — increase the stick count for long runs that need laps.
- Rebar weight
- Rebar weight from the formula above, expressed in lb before metric conversion. Bar weights are published ASTM A615 nominal weights. Lap splices (typically 40–60 bar diameters) and edge clearance are not added — increase the stick count for long runs that need laps.
Frequently asked questions
Eleven bars each way × 10 ft = 220 linear ft, which is eleven 20 ft sticks before laps and cutting waste.
#4 (½ in) weighs 0.668 lb per foot, so a 20 ft stick is about 13.4 lb. #3 is 0.376 lb/ft and #5 is 1.043 lb/ft (ASTM nominal weights).
Residential slabs commonly use #3 or #4 bars at 12–24 in each way, or welded wire mesh. Structural slabs follow the engineer's schedule — this tool estimates quantity, not design.
A common rule is 40–60 times the bar diameter (so 20–30 in for #4), set by your code and engineer. Add lap length when a run exceeds one stick.
Bars are chaired to the engineer's specified cover — often near the lower third for slabs on grade, never lying on the dirt where they cannot develop bond.