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Both directionsLength + weightFree

Rebar Count
Calculator

Estimate one rectangular reinforcing mat in two directions, including total linear length and nominal weight. You supply the layout dimensions and spacing; this tool does not design reinforcement.

Who it's for: estimators and builders converting an already designed rectangular bar layout into a quantity takeoff at construction documents.

Bars = ceil((span − 2·edge)/spacing) + 1
Bars one way = ceil((perpendicular span − 2 × edge clearance) / spacing) + 1. Each bar's length = its span − 2 × cover. Weight uses standard bar unit weights (e.g. #4 = 0.668 lb/ft, #5 = 1.043 lb/ft).
Rebar
Count · length · weight

How to use this calculator

1
Set units and slab size
Pick feet (pounds) or metres (kilograms), then enter the slab length and width. These are the two plan dimensions of the reinforcing mat.
2
Enter bar spacing each way
Type the center-to-center spacing for bars running in each direction. A 6 in spacing is 0.5 ft; a 200 mm spacing is 0.2 m.
3
Set edge clearance and end cover
Edge clearance is how far the first bar sits in from the slab edge; end cover is the concrete over each bar end. A common value is 2 in (0.167 ft).
4
Choose the bar size
Select the bar designation (#3 to #11, or 10M to 36M). The size sets the unit weight used for the tonnage figure.
5
Read count, length, and weight
The panel returns bars each way, total bar count, total linear length, and reinforcement weight for one mat.

The formula

The bar count in each direction divides the net slab run (span minus two edge clearances) by the center-to-center spacing, rounds up to a whole bar, then adds one for the bar at the far edge.

n = ⌈(span − 2 × edge) / s⌉ + 1

n — number of bars in that direction
span — slab dimension perpendicular to those bars (ft or m)
edge — edge clearance, first bar center to slab face (ft or m)
s — center-to-center bar spacing (ft or m)

Bar cut length = span − 2 × end cover. Total linear length = n₁ × length₁ + n₂ × length₂. Weight = total linear length × bar unit weight (e.g. #4 = 0.668 lb/ft per CRSI).

Worked example

Example
On a 20 × 14 ft slab with 6 in (0.5 ft) spacing both ways, 2 in edge clearance and 2 in end cover, bars along the length number ceil((14 − 0.33) / 0.5) + 1 = 29 and bars along the width number ceil((20 − 0.33) / 0.5) + 1 = 41, for 70 bars. Total linear length is 1,130.6 ft, and at #4 (0.668 lb/ft) the mat weighs about 755 lb.

When this estimate will be off

  • Assumes a flat, rectangular slab with uniform spacing — L-shaped or stepped footprints must be split into rectangular sub-panels and the results summed.
  • Does not include lap splices, hooks, stirrups, or construction waste — add 10–15% to the linear-length figure before placing a steel order.
  • Bar unit weights follow CRSI standard values; actual delivered weight may differ by up to ±3.5% per ASTM A615 rolling tolerances.
  • Calculates one mat only (both directions in one layer) — slabs with top and bottom reinforcement need a separate run for each layer.

Frequently asked questions

A common mistake: Assumes a flat, rectangular slab with uniform spacing — L-shaped or stepped footprints must be split into rectangular sub-panels and the results summed.

Bar count is the clear run divided by the spacing, rounded up, plus one. For a 14 ft span with 6 in (0.5 ft) spacing and 2 in edge clearance at each end, that is ceil(13.67 / 0.5) + 1 = 29 bars. The same formula runs in the perpendicular direction.

Dividing span by spacing counts the gaps between bars, not the bars. A run with 28 gaps holds 29 bars, because a bar sits at both ends of every gap. Skipping the plus-one undercounts by one bar in each direction and underestimates the steel quantity.

Edge clearance shortens the span used for counting: the first bar starts a set distance in from each edge, so the tool subtracts twice the clearance before dividing. End cover trims each bar, deducting twice the cover from the full span so steel does not reach the formwork.

Total length is the count in each direction times each bar cut length, summed both ways. Multiply that footage by the bar unit weight for tonnage: a #4 bar weighs 0.668 lb/ft and a #5 weighs 1.043 lb/ft. Add laps, hooks, and waste before ordering.

Sources

  • ACI 318-19 §20.6.1 — Minimum concrete cover requirements for nonprestressed cast-in-place reinforcement in slabs.
  • CRSI Design Handbook — Standard bar unit weights: #3 = 0.376 lb/ft through #11 = 5.313 lb/ft.

This free Rebar Mat Quantity Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.