Free calculator
Rebar Calculator
Enter a slab to get bars each way, laps, sticks to buy, weight and chairs for one or two mats. Add your own prices to get the installed cost, then add the slab to an estimate and do the next one.
- Bars, laps, sticks, chairs
- Your prices, saved
- Builds an estimate
- No email required
Slab
Bars
Lap and chair spacing are defaults, not code values: 48 bar diameters (24 in on #4) and chairs every 3 ft each way, as in our concrete estimate template. Use the lap on your plans and the support spacing in your spec.
Your prices
Optional. Saved in this browser, so you only type them once.
Your estimate
Saved in this browser. Add rooms or runs from any of our calculators.
Nothing here yet. Run the numbers above and press “Add to estimate”.
Ready to send this as a proposal?
BuildWorkPro builds bids from line items like these, with your margin and overhead, and turns them into a PDF proposal. $79/mo flat, free for 14 days.
Start free trialHow it’s calculated
Clear span. Each bar runs the slab dimension less the edge clearance at both ends: length in inches − 2 × clearance. The 3 in default is the clearance in our concrete estimate template. Your plans set the cover; ACI 318 Table 20.5.1.3.1 gives the minimums by exposure.
Bars each way. Bars along the length are spaced across the width: ROUNDUP(clear width ÷ spacing) + 1, so there is a bar at each edge. Bars along the width are ROUNDUP(clear length ÷ spacing) + 1. A top mat doubles both.
Laps. The lap is the number of bar diameters you enter × the bar’s nominal diameter. A run longer than one stick takes n = ROUNDUP((span − lap) ÷ (stick − lap)) pieces, at least one, with n − 1 laps, so its steel is span + (n − 1) × lap. Linear feet are bars × steel per run, both ways. This is the same grid math as the concrete estimate template’s slab takeoff.
Sticks. Counted from a cut list rather than linear feet ÷ stick length. Each spliced run takes n − 1 full sticks and a last piece. The last pieces are fitted onto sticks longest first, and short pieces go into the offcuts of long ones where they fit. Your waste percentage goes on top, rounded up. Offcuts are not spliced into new bars, so a cut plan made by hand can sometimes save a few sticks.
Weight. Linear feet × the nominal weight per foot from ASTM A615, and tons = lb ÷ 2,000. The weight placed counts the laps; the weight bought is every stick you buy, offcuts included.
| Bar | Diameter (in) | lb per ft | 20 ft stick (lb) | Lap at 48 diameters |
|---|---|---|---|---|
| #3 | 0.375 | 0.376 | 7.52 | 18 in |
| #4 | 0.500 | 0.668 | 13.36 | 24 in |
| #5 | 0.625 | 1.043 | 20.86 | 30 in |
| #6 | 0.750 | 1.502 | 30.04 | 36 in |
| #7 | 0.875 | 2.044 | 40.88 | 42 in |
| #8 | 1.000 | 2.670 | 53.4 | 48 in |
Chairs. ROUNDUP(slab area ÷ spacing²) for each mat. The default, 3 ft each way or one chair per 9 sq ft, is the concrete estimate template’s figure, not a code value.
Price. Steel is sticks × your price per stick, or the weight of the sticks you buy × your price per ton. Chairs are the count × your price each. Labor is tons placed × your rate per ton, or slab area × your rate per sq ft. Tie wire is not counted, because the number of ties depends on the spec. Nothing is priced with numbers you did not enter.
Bar size, spacing, cover and laps are design decisions that come from the drawings and the engineer, and this calculator does not check them. Pricing the concrete too? The concrete estimate template has a slab takeoff for cubic yards, base stone, rebar and forms, and the concrete block calculator counts blocks, grout and bars for a CMU wall.
Keep going
More free tools and price data: Concrete Block Calculator · Asphalt Calculator · Markup vs Margin Calculator · Steel and Rebar Prices
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
With #4 bars at 12 inches on center each way, 3 inches of edge clearance and 20 ft sticks, a 20 × 20 ft slab takes 21 bars each way, 42 bars of 19 ft 6 in. Every bar fits in one stick, so there are no laps: 819 linear feet, 547 lb (0.27 tons) of steel and 42 sticks, plus 45 chairs at 3 ft each way. Buy 40 ft sticks and it is 21, two bars to a stick. Use the bar size and spacing on your plans.
How much does rebar weigh per foot?
ASTM A615 gives the nominal weights: #3 is 0.376 lb per foot, #4 0.668, #5 1.043, #6 1.502, #7 2.044 and #8 2.670. A 20 ft stick of #4 weighs 13.36 lb and one of #5 20.86 lb. A ton of #4 is about 2,994 feet, or 149.7 sticks of 20 ft.
Why does it show more sticks than the linear feet divided by 20?
Because steel comes in whole sticks. The 20 × 20 slab above is 819 LF, which is 40.95 sticks of steel, but each 19 ft 6 in bar needs its own 20 ft stick and the offcuts are scrap, so you buy 42. The calculator builds a cut list: a full stick for every piece of a spliced run but the last, then the last pieces fitted onto sticks longest first, with short pieces cut from the offcuts of long ones where they fit. On the 40 × 30 ft example slab that is 144 sticks against 132 by straight division.
How far apart should rebar be in a concrete slab?
As far apart as the plans say. Bar size and spacing are design decisions that depend on the slab’s thickness, loads and subgrade, and on how much cracking is acceptable. For structural slabs, ACI 318 also caps the spacing based on the slab thickness (sections 7.7.2.3, 8.7.2.2 and 24.4.3.3). Enter the spacing from your drawings or spec. It can differ each way.
How long should rebar laps be?
Use the lap on your plans. Under ACI 318 the engineer shows the location and length of lap splices on the structural drawings, and the length depends on concrete strength, bar grade, size and spacing (CRSI). A tension lap is 1.0 or 1.3 times the bar’s development length (ACI 318 Table 25.5.2.1). The calculator takes the lap in bar diameters and defaults to 48, which is 24 in on #4 bar, the lap in our concrete estimate template. At 48 diameters a #3 laps 18 in, a #5 30 in and a #6 36 in. Change it to match your spec.
How many rebar chairs do I need?
One for each chair-spacing square of slab, per mat: slab area ÷ (spacing × spacing), rounded up. At 3 ft each way that is one per 9 sq ft, so a 20 × 20 slab takes 45 and the 40 × 30 example takes 134. A top mat needs its own supports, counted at the same spacing. Supports have to hold the steel at its depth through the pour (2021 IRC R506.2.4 for slabs on ground), so follow your spec, which may call for closer supports under heavier bars.
Are my measurements and prices sent to BuildWorkPro?
The calculator sends nothing: the math runs in your browser, and your prices and your estimate are saved in this browser’s local storage, never on our servers. The slab size is kept in the page address so you can bookmark or share a result; like any page address, it is recorded by our site analytics if you accept analytics cookies.