Work out how much rebar a slab needs from its size and bar spacing. Bar counts each way, total length, weight and stock lengths.
A single flat mat in a rectangular slab.
Enter the slab size and spacing to see the bar count.
All calculations run in your browser — nothing is sent anywhere.
bars = ⌊(span − 2 × cover) ÷ spacing⌋ + 1
Work each direction separately. Take the span, subtract the cover at both edges, divide by the bar spacing and round down, then add one for the closing bar. Multiply each count by that bar's length and add the two directions together.
Worked example: A 20 × 10 ft slab at 12 in spacing with 3 in cover takes 10 bars of 19.5 ft one way and 20 bars of 9.5 ft the other — 385 ft in total, about 257 lb of
Rebar is ordered by length and delivered by weight, but designed by spacing — which makes it easy to order the wrong amount. This calculator takes a slab’s dimensions and your specified spacing and returns the bar count each way, the total linear length, the weight, and how many stock lengths that comes to.
It covers a single flat mat in a rectangular slab, which is the common residential case. Thickened edges, beam strips, extra bars around openings and second mats all need adding separately.
For a 20 × 10 ft slab at 12 in centres with 3 in cover:
| Count | Bar length | Total | |
|---|---|---|---|
| Running lengthwise | 10 | 19.5 ft | 195 ft |
| Running widthwise | 20 | 9.5 ft | 190 ft |
| Total | 30 bars | 385 ft |
At #4 that is about 257 lb, or 20 × 20 ft stock lengths before cutting waste.
Note how the counts invert: bars that run the length are spaced across the width, so the narrow dimension sets how many long bars you need. Getting these the wrong way round is the most common arithmetic slip in rebar take-offs.
| US bar | Diameter | Weight |
|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft |
| #4 | 1/2 in | 0.668 lb/ft |
| #5 | 5/8 in | 1.043 lb/ft |
| #6 | 3/4 in | 1.502 lb/ft |
| Metric bar | Weight |
|---|---|
| 10 mm | 0.617 kg/m |
| 12 mm | 0.888 kg/m |
| 16 mm | 1.579 kg/m |
| 20 mm | 2.466 kg/m |
The calculator does not add lap splices, because the right lap length is not something to guess at. It depends on bar size, concrete strength, whether the bar is coated, and the code in force — commonly in the region of 40 bar diameters, which is about 20 in for a #4, but that figure is an illustration and not a specification.
Any slab longer than your stock bars needs splices, and each one consumes a lap length of bar. Count the splices your layout requires, multiply by the specified lap, and add it before ordering.
The calculator does what you tell it. What it cannot tell you is whether the spacing is right.
Bar size, spacing, cover and slab thickness follow from the loads, the soil and the code that applies where you are building. Twelve-inch centres with #4 bar is a common residential arrangement, not a universal answer, and a driveway carrying delivery trucks is not the same problem as a garden shed base. Take these from the drawings or from an engineer.
What the calculator will stop you doing is ordering for the wrong geometry — which is where most of the money gets wasted.
Work out the concrete itself with the cubic yard calculator or the rectangular slab calculator, then price it with the concrete cost calculator. For access planning, the concrete weight calculator tells you what the pour will weigh.
Pouring a different shape? These cover the rest of the common jobs.
Calculate the concrete volume for a rectangular slab from length, width and depth. Imperial and metric units, with a 10% waste buffer per ACI 318.
Calculate the concrete volume for a circular slab from diameter and depth. Imperial and metric units, with a 10% waste buffer per ACI 318.
Calculate the concrete volume for an irregular slab from a pre-measured area and depth. Imperial and metric units, with a 10% waste buffer per ACI 318.
Calculate the concrete volume for a circular column from diameter and height. Imperial and metric units, with a 10% waste buffer per ACI 318.
Calculate the concrete volume for fence-post holes — by hole, total across all holes, with post-profile displacement and a 10% waste buffer.
This tool is web-only. The Concrete Calculator app covers the five volume calculators — rectangular and circular slabs, irregular slab area, circular columns and post holes — with saved projects, job history and full offline use.
Free · 4.8 ★ on the App Store
