Rebar takeoff for a slab, laps included.
Enter the slab, spacing, cover and bar size. The calculator counts bars each way, adds a lap wherever a run is longer than your stock length, and returns linear feet, weight and stock bars. Lap length is an editable assumption: use the one on the structural drawings.
- #3 to #11 bars
- Laps at stock length
- One or two mats
- Weight in lb & tons
Slab or footing
Bars
Stock, laps & waste
Save your results
Copy these results, then add your approved sample files in the trial brief.
Quantities for estimating only. Lap lengths, cover, bar supports and hooks come from the structural drawings and ACI 318 as adopted locally.
Rebar weights by bar size, ASTM A615.
The bar number is the nominal diameter in eighths of an inch (#4 = ½"). Weight per foot is what turns linear feet into tons.
| Bar | Diameter | lb/ft | kg/m |
|---|---|---|---|
| #3 | 0.375" | 0.376 | 0.560 |
| #4 | 0.500" | 0.668 | 0.994 |
| #5 | 0.625" | 1.043 | 1.552 |
| #6 | 0.750" | 1.502 | 2.235 |
| #7 | 0.875" | 2.044 | 3.042 |
| #8 | 1.000" | 2.670 | 3.973 |
| #9 | 1.128" | 3.400 | 5.060 |
| #10 | 1.270" | 4.303 | 6.404 |
| #11 | 1.410" | 5.313 | 7.907 |
How the calculator counts bars.
Bars each way
Clear width (width minus cover on both sides) divided by spacing, rounded down, plus one bar to close the far edge.
Run length
Each bar runs the clear length. If that exceeds your stock length, it is spliced.
Laps
Pieces = ceil((run − lap) ÷ (stock − lap)). Each splice adds one lap length of steel.
Weight
Total length plus waste, times lb/ft for the bar size. Stock bar count is before cut optimization.
What a slab calculator does not see.
Use the number as a check against your detailed takeoff, and add these separately.
- Hooks and bends. Add standard hook lengths at discontinuous edges where the details call for them.
- Openings and thickened edges. Trim bars at openings and add any trim bars shown in the details.
- Chairs and supports. Bar supports are a separate line item.
- Dowels and corner bars. Count these from the details; they never show up in a grid calculation.
- Fabrication extras. Detailing, cutting and bending charges are usually priced by the fabricator.
Calculators check one number. Bids need every quantity.
For full concrete sets, RenderDraw proposes quantities from the plans and details with the source sheet attached, and your estimator approves them before pricing.
- Source on every row. Each proposed quantity keeps the sheet and location it came from.
- Estimator approval before pricing. Your estimator accepts, corrects or rejects proposals; nothing is priced until it is approved.
- Addenda through the same review. Changed sheets go back through takeoff and estimator review.
- Your assemblies, your rates. Approved rows hand off to your assemblies, labor units and pricing inputs.
Rebar calculator, answered.
How do I calculate how much rebar I need for a slab?
Count bars each way: (clear width ÷ spacing) + 1. Multiply each count by the bar run length, add lap splices where a run is longer than the stock bar, then add waste. Multiply the total length by the bar's weight per foot to get pounds.
How long should rebar laps be?
It depends on bar size, concrete strength, cover, spacing and the splice class in ACI 318, so take it from the structural drawings or lap schedule. The calculator's 40 bar diameters is only a placeholder you should replace.
How much does #4 rebar weigh per foot?
0.668 lb/ft under ASTM A615. #5 is 1.043 lb/ft and #6 is 1.502 lb/ft.
How do I convert rebar linear feet to tons?
Multiply linear feet by the bar's lb/ft, then divide by 2,000. For example, 3,000 LF of #4 is 3,000 × 0.668 ÷ 2,000 ≈ 1.0 ton.
Does this include hooks, dowels and chairs?
No. It covers the straight-bar grid for a rectangular slab or footing. Add hooks, dowels, trim bars and supports from the details.
Send one plan set. Judge the result yourself.
A focused, no-obligation evaluation on your own drawings. Your sample files remain private while we assess the request.
- Upload one representative plan set (a legible PDF set is the most reliable starting point), plus an addendum if you have one.
- RenderDraw reviews the set and the scope you want tested.
- We follow up with what was extracted, what was flagged for review, and the most useful next step.