Rebar & Mesh Calculator
A 20 × 30 ft slab with #4 bar at 12 inches on centre and 3 inches of cover takes 20 bars one way and 30 the other — 70 × 20 ft lengths including 20 inch lap splices. Note the plus-one: a 20 ft span at 12 inch spacing takes 21 bars, not 20. Bar weights per ASTM A615, cover per ACI 318, verified 2026-08-04.
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Rebar & Mesh Calculator — materials list
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| Item | Quantity | Cost |
|---|
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Prepared with MaterialMath — https://estimatormath.com. Quantities are estimates based on the dimensions entered and the published figures cited on the tool page (verified 2026-08-04). Verify against your own measurements and your supplier's actual pack sizes before ordering. Not professional engineering, construction or financial advice.
How is a rebar schedule calculated?
A slab mat is two sets of parallel bars at right angles. Each set runs the full clear dimension in one direction and is spaced across the clear dimension in the other. The whole calculation follows from that, plus lap splices where a run is longer than a stock bar.
The + 1 is the classic error. Think of a fence: 20 feet of
fence at 12 foot spacing has posts at 0, 12 and 24 — you count the
intervals and then add the one at the start. A 20 ft span at 12 inch bar
spacing takes 21 bars, and getting it wrong in both directions on a
large slab leaves you two bars short on the day.
A worked example: a 20 × 30 ft garage slab
#4 bar at 12 inches on centre, 3 inches of cover (cast against earth), 20 ft stock lengths.
Step by step
- Slab area
- 600 ft²
- Clear dimensions after 3″ cover each side
- 29.5 × 19.5 ft
- Bars running the length
- 19.5 ft ÷ 12″ + 1 = 20 bars
- Bars running the width
- 29.5 ft ÷ 12″ + 1 = 30 bars
- Lap splice length
- 40 × 0.5″ = 20″
- Total stock bars to buy
- 70 × 20 ft lengths
- Intersections to tie
- 600
The lengthwise bars run 29.5 ft, which is longer than a 20 ft stock bar, so each one is two pieces with a 20 inch lap between them. That lap is not waste — it is what makes the two pieces act as one bar, by giving the concrete enough bonded length to transfer the force from one to the other.
Bar sizes and weights
Click a column to sort — for example by weight, to compare bar sizes at a glance.
| Bar | Diameter | lb per ft | kg per m | 40d lap | Metric |
|---|---|---|---|---|---|
| #3 (3/8") | 0.375″ | 0.376 | 0.559 | 15″ | 10M |
| #4 (1/2") | 0.500″ | 0.668 | 0.994 | 20″ | 13M |
| #5 (5/8") | 0.625″ | 1.043 | 1.552 | 25″ | 16M |
| #6 (3/4") | 0.750″ | 1.502 | 2.235 | 30″ | 19M |
| #7 (7/8") | 0.875″ | 2.044 | 3.042 | 35″ | 22M |
| #8 (1") | 1.000″ | 2.67 | 3.973 | 40″ | 25M |
Bar number is the diameter in eighths of an inch — #4 is 4/8", or half an inch. Weights are the ASTM A615 nominal figures; mill tolerance permits up to 6% under.
Concrete cover by exposure
| Condition | Minimum cover |
|---|---|
| Cast against and permanently exposed to earth | 3" |
| Exposed to earth or weather (#6 and larger) | 2" |
| Exposed to earth or weather (#5 and smaller) | 1-1/2" |
| Not exposed (interior slab) | 3/4" |
Per ACI 318. Cover is what keeps water away from the steel — too little and the bar rusts, expands, and spalls the concrete off the face.
Who this calculator is for
- Anyone pouring a slab-on-grade — garage, shed, driveway, patio — who needs a bar count and a tie-wire estimate.
- Concrete contractors doing a quick takeoff before ordering steel.
- People comparing rebar against mesh. Switch the mode and the same slab is priced both ways, including the chairs that mesh needs more of than rebar does.
- Owner-builders checking a supplier's quote against their own drawing.
The concrete for the slab itself comes from the concrete calculator, and the compacted base underneath it from the gravel calculator — remember that compactable base settles about 20%.
What this calculator does not account for
- Engineering. This produces a bar schedule for a rectangular crack-control mat. It does not design reinforcement. Bar size, spacing and cover for anything structural come from an engineer, and their schedule replaces this one entirely.
- Openings and thickened edges. Slabs with a thickened perimeter, a grade beam, or an opening for a drain all need extra steel that a rectangular mat calculation cannot see.
- Dowels and connections. Doweling into an existing slab or a foundation wall is separate, and so are the vertical bars in any stem wall.
- Corner and edge bars. Many drawings call for extra bars around the perimeter or diagonal bars at re-entrant corners, which is exactly where slabs crack.
- Class B tension laps. The 40d rule here is the residential slab convention. ACI 318 structural laps depend on concrete strength, coating, cover and spacing and can be much longer.
- Fibre reinforcement. An alternative to mesh for crack control in some slabs, dosed by the cubic yard rather than laid out.
Welded wire mesh styles
| Style | Weight per 100 ft² | Supplied as |
|---|---|---|
| 6×6 W1.4/W1.4 (10 ga) | 21 lb | 5' × 10' sheet or 5' × 150' roll |
| 6×6 W2.9/W2.9 (6 ga) | 43 lb | 5' × 10' sheet or 5' × 150' roll |
| 6×6 W4.0/W4.0 (4 ga) | 59 lb | 5' × 10' sheet |
| 4×4 W1.4/W1.4 (10 ga) | 31 lb | 5' × 10' sheet or 5' × 150' roll |
Mesh sheet counts include 12% for the lap, which must be one full mesh square and at least 6 inches. Sheets are far easier to place flat than rolls, which want to curl back up.
Common questions
How much rebar do I need for a slab?
Count the bars in each direction, then multiply by the run length. Bars running the length are spaced across the width, and vice versa. A 29.5 × 19.5 ft clear area at 12 inch spacing takes 20 bars one way and 30 the other.
The detail people miss is the +1. A 20 ft span at 12 inch spacing takes 21 bars, not 20 — there is a bar at each end of the span as well as at every interval. Miss it in both directions on a big slab and you are two bars short.
What size rebar for a 4 inch concrete slab?
#3 or #4 at 12 to 18 inches on centre is typical for a residential slab-on-grade. #4 (½ inch) at 12 inches is the common default and is what this calculator starts with.
Be clear about what it is doing: reinforcement in a slab-on-grade controls cracking. It does not make the slab structural. Anything carrying a wall, a column or vehicle loads needs an engineer, and the engineer's bar schedule replaces this one.
How long should a rebar lap splice be?
40 bar diameters is the common slab-on-grade convention and what most residential plans call for. For #4 bar that is 40 × 0.5" = 20 inches.
Structural work is different. ACI 318-19 Class B tension laps are computed from concrete strength, bar coating, cover and bar spacing, and can be considerably longer than 40d. Where there is an engineer's drawing, its lap length replaces this rule.
How much concrete cover does rebar need?
Per ACI 318: 3 inches where concrete is cast against and permanently exposed to earth, 2 inches where exposed to earth or weather for #6 bar and larger, 1½ inches for #5 and smaller, and ¾ inch for interior slabs not exposed.
Cover is what keeps water and chlorides away from the steel. Too little and the bar rusts; rust occupies more volume than steel, so it spalls the concrete off the face. Once that starts it does not stop.
How much does rebar weigh?
#3 is 0.376 lb per foot, #4 is 0.668, #5 is 1.043, #6 is 1.502. The pattern is roughly that weight scales with the square of the diameter, which is why going up one bar size adds far more steel than it looks.
These are the ASTM A615 nominal figures. Mill tolerance permits actual weight up to 6% under nominal, so a load that weighs light at the scale is not necessarily short.
Is welded wire mesh as good as rebar?
For crack control in a residential slab, mesh is adequate and cheaper. For anything structural, it is not — mesh has far less cross-sectional area and no meaningful bending capacity.
The bigger problem is installation. Mesh only works if it sits in the middle third of the slab depth. Mesh laid on the ground and "hooked up" with a rake during the pour ends up on the bottom, where it does nothing at all. If you use mesh, use chairs, and enough of them that it does not walk flat when someone stands on it.
Sources for the figures used
Every rate, coverage figure and code limit in this calculator comes from a published source. Where a figure is a convention rather than a standard, it says so.
- ASTM A615/A615M — Standard Specification for Deformed and Plain Carbon-Steel Bars — Bar designations, nominal diameters and unit weights (checked 2026-08-04)
- Concrete Reinforcing Steel Institute (CRSI) — Bar size tables, placing practice, chair and bolster spacing (checked 2026-08-04)
- ACI 318-19 — Building Code Requirements for Structural Concrete — Concrete cover requirements and lap splice provisions (checked 2026-08-04)
- Welded wire reinforcement weights — Standard WWR styles and weights per 100 ft²; lap requirement of one full mesh square, 6" minimum (checked 2026-08-04)