Rebar Calculator

How much rebar do I need? A 20 ft x 12 ft concrete slab on a 12-inch grid with 3-inch edge cover needs 495 linear feet of bar: 13 bars running the long way plus 21 running the short way, or 25 standard 20-foot sticks (28 sticks with 10% waste), about 331 lb of #4 bar. Get that number wrong and you stall the pour waiting on a steel run or truck back a half-bundle you never used. This rebar calculator turns two slab dimensions and a spacing into an exact bar count, total linear feet, and a clean shopping list of 20-foot sticks.

This tool sizes only the steel grid inside a slab. Pair it with the Concrete Calculator for the cubic yards of concrete that go around the bar, and if the same project has a roof, the Roofing Calculator and Roof Pitch Calculator size shingles and slope while the Paint Calculator handles wall and floor coatings.

How the rebar grid is calculated

A slab is reinforced with a two-way grid: one set of bars runs the length of the slab, a second set runs the width, and they cross to form a mat. The number of bars on each axis is set by the spacing and how far you keep the steel in from the edges:

The +1 matters: if a 12 ft run is spaced every 12 inches you get 12 gaps but 13 bars, because there is a bar at both ends of the run. Rounding up means that when the span does not divide evenly, the bars are set slightly closer than your spacing rather than leaving a wider gap at one edge. Once you have the count for each direction, the total length of steel is:

Here c is the edge clearance, because each bar stops at the cover line. Divide the total by your stick length (20 ft is a common stock length) and round up to get the number of bars to order.

Worked example: a 20 x 12 ft slab

Take a 20 ft long, 12 ft wide slab, rebar at 12 inches on center, with 3 inches of edge clearance all around. The usable span is the slab minus 6 inches (3 in on each edge) per axis, so 19.5 ft and 11.5 ft. That gives ceil(11.5 / 1) + 1 = 13 bars running the long direction and ceil(19.5 / 1) + 1 = 21 bars running the short direction. Each long bar is 19.5 ft and each short bar 11.5 ft, so linear feet are 13 x 19.5 = 253.5 ft plus 21 x 11.5 = 241.5 ft, for 495 linear feet. At 20 ft per stick that is 25 sticks, or 28 sticks with a 10% overage. In #4 bar it weighs 495 x 0.668 = about 331 lb. The grid has 13 x 21 = 273 intersections to tie.

How to measure and use this calculator

You only need a tape measure and your plans. Work through the fields in order:

1. Slab length and width: measure the finished slab footprint in feet. For an irregular shape, break it into rectangles and run each one separately.
2. Bar spacing: enter the center-to-center spacing in inches from your plans (commonly 12, 16, or 18 inches). The preset buttons cover the usual values.
3. Edge clearance: the concrete cover that keeps steel away from the slab edge; the IRC foundation rules call for 3 inches where concrete is cast against the earth.
4. Stick length: the length of the bar your supplier sells, often 20 ft.
5. Bar size: #3 to #6 from your plans; it only changes the weight.
6. Waste / overage: nudge the slider up for laps and cuts; 10% is a sensible default.

The results update as you type: read the big linear-foot number at the top, then the "what to buy" card for the stick count and weight to order.

Who this calculator is for

• DIY slab pourers sizing rebar for a shed floor, patio, or garage slab.
• Homeowners sanity-checking a contractor's material list before paying for it.
• Contractors and estimators who want a fast linear-foot and stick count for a takeoff.
• Landscapers reinforcing driveways, walkways, and equipment pads.
• Anyone buying rebar who needs to translate a grid spacing into bundles to load on the truck.

Key rebar terms explained

• Linear feet: the total running length of bar, regardless of size: the figure you buy by (or convert to weight).
• On center (OC): spacing measured from the center of one bar to the center of the next, the standard way spacing is given.
• Concrete cover / clearance: the thickness of concrete between the steel and the surface or edge, which protects the bar from rust.
• Lap splice: the overlap where two bars join end to end so the load transfers; it consumes extra length, set by your plans for the bar size.
• Mat: a complete two-way layer of crossing bars; some slabs use one mat, heavier ones use two.
• Waste factor: the percentage you add on top of the exact figure for cuts, laps, damage, and miscounts.
• Cubic yard: the unit concrete is sold in (27 cubic feet); use the Concrete Calculator alongside this tool to order the mud that goes around the steel.

Rebar size chart: #3, #4 and #5 diameter and weight

Bar size does not change the counts above; it changes strength and weight. The number is the diameter in eighths of an inch (#4 = 4/8 = 1/2 inch). Weights are the standard ASTM nominal values; use them to turn linear feet into pounds for delivery.

So the 495 linear feet from the worked example weighs about 186 lb in #3 bar, 331 lb in #4, or 516 lb in #5; the count is the same, only the tonnage differs.

Scenario 1: a 10 x 10 ft patio at 16-inch spacing

A small 10 x 10 ft patio at 16-inch (1.33 ft) spacing with 3-inch cover has a 9.5 ft (114 in) usable span each way. That is ceil(114 / 16) + 1 = 9 bars in each direction, set 14.25 inches apart. Linear feet are 9 x 9.5 + 9 x 9.5 = 171 ft, or 9 sticks of 20-foot bar, 10 with a 10% overage. Wider spacing means fewer bars, which is why a patio uses far less steel than a structural slab of the same size.

Scenario 2: a 24 x 24 ft garage slab at 12-inch spacing

A two-car garage at 24 x 24 ft on a tight 12-inch grid with 3-inch cover has a 23.5 ft usable span each way, giving ceil(23.5 / 1) + 1 = 25 bars per direction. Linear feet are 25 x 23.5 + 25 x 23.5 = 1,175 ft, which is 59 sticks, or 65 with a 10% overage. Note how much faster steel adds up on a square slab when both dimensions and the grid are large.

Scenario 3: the same slab, looser spacing

Take that 24 x 24 ft slab and open the grid to 18 inches (1.5 ft). The count drops to ceil(23.5 / 1.5) + 1 = 17 bars per direction, for 17 x 23.5 + 17 x 23.5 = 799 ft, about a third less steel than the 12-inch grid. Spacing is the biggest single lever on rebar quantity, which is exactly why it should come from your engineer or code, not from rounding to save a few sticks.

How much rebar do I need? Quantities by slab size

Common slab footprints on a 12-inch grid with 3-inch cover, computed with the formula above (bars stop at the cover line). "Sticks" is the number of 20-foot bars to buy including a 10% waste allowance; "ties" is the grid intersection count for sizing tie wire.

Loosen the grid and every figure drops: a 20 x 20 ft slab needs 819 ft at 12 inches, 624 ft at 16 inches, 546 ft at 18 inches, and 429 ft at 24 inches. Take spacing from your plans, not from this table.

What changes the result the most

• Spacing: the dominant factor; halving the spacing roughly doubles the bar count.
• Slab size: linear feet scale with both dimensions, so a larger slab grows steel quickly.
• Edge clearance: it shortens every bar and can change the count by one on small pours.
• Stick length: changes how many bars to buy and how many laps you need, not the linear-foot total.
• Waste factor: the gap between the exact figure and what you actually order.

Tips for ordering rebar

• Always order about 10% extra. Laps, cuts, and the odd bent bar eat into the exact figure fast.
• Account for lap splices when a bar must run longer than one stick; each splice adds the lap length shown on your plans.
• Buy chairs or dobies to hold the mat at the right height; the IRC wants slab reinforcement between the center and the upper third of the slab.
• Get tie wire for the intersections; the calculator gives the intersection count to size that.
• Match bar size to your plans (#3, #4, #5). Size does not change the count, but it changes strength and weight.

Limitations and assumptions

• It models a single rectangular two-way mat; for two mats, run it twice and add the totals.
• The base linear feet assume continuous bars; it does not add lap-splice length automatically (use the waste slider).
• It does not choose bar size, cover depth, or slab thickness, or check that the design meets code; the bar size only sets the weight.
• It excludes chairs, tie wire, dowels, and edge/perimeter bars beyond the grid.
• It is a material estimate, not a structural design; follow engineered drawings for anything load-bearing.

Related materials and calculators

Rebar is only one part of a slab takeoff. To finish the job, pair this with a few sister tools:

• Use the Concrete Calculator to find the cubic yards of concrete that go around the steel.
• Use the Square Footage Calculator to confirm the slab area before you lay out the grid.
• Use the Cubic Yard Calculator for any base or fill material under the slab.
• Use the Gravel Calculator to size the sub-base the slab is poured on.
• Use the Mulch and Paint calculators for the rest of the project's material list.

Sources

• ASTM International: ASTM A615/A615M, Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement (Table 1 nominal diameters and weights).
• International Code Council: 2021 International Residential Code, Chapter 4 (concrete cover: 3 in where cast against the earth).
• International Code Council: 2021 International Residential Code, Section R506 (concrete slabs-on-ground, reinforcement support).