Concrete calculator
Rebar Calculator
Enter slab dimensions, first-bar inset, grid spacing, bar size, stock length, and allowance to estimate grid counts, total linear feet, stock-bar equivalents, and steel weight.
Planning estimate, not a contractor quote. Review the assumptions and replace presets with local prices when available.
Method
How this calculator works
This calculator performs a rectangular two-direction rebar takeoff. It places a bar on both inset boundary lines and adds enough equal intervals that no interval exceeds the entered spacing. The entered bar layout must come from project drawings or qualified design.
Clear grid dimension = slab dimension − 2 × first-bar inset. Bars in a direction = ceiling(perpendicular clear dimension ÷ spacing) + 1. Total linear feet = length-direction bars × clear length + width-direction bars × clear width. Allowance, stock-bar equivalent, and nominal weight are then calculated.
Worked example
A realistic starting point
For a 20 ft by 20 ft slab with first-bar centerlines 3 in from each edge and 18 in maximum spacing, the clear grid is 19.5 ft each way and needs 14 bars per direction. Raw length is 546 ft; with 10% allowance it is 600.6 ft, equal to 31 theoretical 20 ft bars and about 401.2 lb of #4 steel.
What changes the answer
Factors worth checking
Start with the drawings
Bar size, spacing, layers, cover, supports, development, and splices are design decisions. This tool only takes off the layout you enter.
Inset is to bar centerline
Enter distance from slab edge to the first and last bar centerlines. Do not substitute code concrete cover without converting it to the required centerline position.
Spacing is a maximum
The calculator rounds the number of intervals up, then includes a bar at both boundary lines. Resulting equal spacing may be slightly less than the entered value.
Stock count is theoretical
Total length divided by stock length does not optimize cuts or design splices. Offcuts, laps, bends, hooks, and supplier lengths can change the purchase plan.
Common questions
Rebar grid FAQ
Why does the formula add one bar?
Spacing describes intervals between bars. If a clear span needs N intervals to stay at or below maximum spacing, it needs N + 1 boundary bars.
What rebar size and spacing should I use?
Use structural drawings or qualified project design. Loads, slab thickness, support, shrinkage control, exposure, and local requirements affect reinforcement.
How is rebar weight calculated?
Adjusted linear feet are multiplied by the selected bar’s nominal pounds per foot. Values match standard U.S. reinforcing-bar properties.
Is the stock-bar number an exact shopping list?
No. It is adjusted total length divided by selected stock length and rounded up. A cut list and approved splice plan are needed for procurement.
Evidence
Sources & review
- VDOT — Reinforcing Steel, Chapter 7Official table of U.S. bar dimensions, areas, weights, and detailing references.
- CRSI StandardsReinforcing-steel authority for design, detailing, fabrication, placement, and construction standards.
- ACI 318 PLUSOfficial access point for current structural-concrete code and detailing resources.
Published by Calcstead. Report a correction.
Method and source links reviewed September 4, 2026. Price presets are editable planning inputs and should be replaced with current local information.