Rebar Quantity Calculator
Slab grid — bars in both directions
Total Steel Weight
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Total Length—
Bars (est.)—
Steel Tons—
Master Reinforcement Estimator
Rebar Calculator
Estimate reinforcement bars, steel quantity, rebar spacing, and concrete slab reinforcement using accurate contractor-grade rebar estimation formulas. Use as a rebar estimator or slab rebar calculator before you tie steel on site.
- Reinforcement Estimation
- Contractor Friendly
- Instant Results
- Mobile Optimized
Rebar Size Chart
Standard deformed bar weights used in this steel reinforcement calculator.
| Bar Size | Diameter | Weight / ft | Common Use |
|---|---|---|---|
| #3 | 3/8 in (9.5 mm) | 0.376 lb | Light slabs, temp mesh alternative in small pours |
| #4 | 1/2 in (12.7 mm) | 0.668 lb | Residential slabs, driveways, footings |
| #5 | 5/8 in (15.9 mm) | 1.043 lb | Garage slabs, heavier footings |
| #6 | 3/4 in (19.1 mm) | 1.502 lb | Structural slabs, grade beams |
| #7 | 7/8 in (22.2 mm) | 2.044 lb | Commercial foundations |
| #8 | 1 in (25.4 mm) | 2.670 lb | Columns, heavy structural members |
Reinforcement Examples
Tap a preset to load dimensions into the rebar calculator for concrete slab and footing layouts.
Garage Slab
24 ft × 24 ft slab, #4 at 12 in OC, 5% waste.
- Layout
- Two-way grid
- Typical steel
- ~400+ lb
Patio Slab
12 ft × 12 ft patio, #3 at 18 in OC.
- Layout
- Light grid
Driveway Slab
20 ft × 40 ft, #4 at 12 in OC.
- Layout
- Two-way grid
Footing Reinforcement
40 ft run × 16 in width footing, #5 at 12 in OC.
- Layout
- Footing grid
Which reinforcement tool should I use?
- This page (Rebar calculator): Full reinforcement modes—slab grid, footing, column, and wall layouts.
- Slab rebar calculator: Patio, garage, and driveway slab steel only.
- Rebar spacing calculator: Spacing verification and bar count.
- Rebar takeoff calculator: Ordering summary with layers and waste.
- Rebar overlap calculator: Lap splice length—not grid takeoff.
Rebar Calculation Formula
This rebar quantity calculator converts all inputs to feet, counts bars across each span, and multiplies by bar run length. For two-way slabs, it adds steel running both directions—the standard reinforcement calculator approach for flatwork takeoffs.
Bars = ⌈ Span ÷ Spacing + 1 ⌉
Total length = Σ (bars × run length) + overlap · (1 + waste%)
Rebar spacing
Spacing is center-to-center distance between parallel bars. Enter the value from your drawing (often 12 in or 18 in on center). The tool divides the span by spacing in feet, adds one bar, and rounds up per your field formula.
Overlap
When bar stock is shorter than the run, splices need lap length. Enter total overlap allowance in inches or feet for the pour—not per bar unless you adjust manually. Structural plans specify lap; this field is for planning cushion.
Steel weight
Total weight = final length × weight per foot for the selected bar size. Use our Concrete Calculator to estimate slab volume before calculating reinforcement, and the concrete yard calculator for supplier-ready cubic yards.
Contractor Tips
- Match the structural detail. A concrete slab rebar calculator gives planning numbers; engineered slabs may require #5 at 10 in OC or fiber supplement—never substitute without approval.
- Chair height and cover. Bars must sit at specified cover (often 1½–2 in under slabs). Chairs add little weight but prevent rock pockets under steel.
- Order length wisely. Compare 20 ft vs 40 ft stock to minimize laps. Add overlap allowance when many splices are expected.
- Mark orientation. On rectangular pours, tag which bars run which way so finishers do not confuse your grid during the pour.
- Pair with volume takeoff. Run the concrete volume calculator and concrete cost calculator for material and budget on the same footprint.
Rebar Calculator FAQ
What spacing should I use for a driveway slab?
Many residential drives use #4 rebar at 12 inches on center each way over compacted base, but frost depth, soil, and local code may require a different layout. Use engineering when in doubt.
How do I count bars across a 20-foot width at 12-inch spacing?
Convert spacing to feet (12 in = 1 ft). Bars = ceil(20 ÷ 1 + 1) = 21 bars across that span for this calculator’s formula. Each bar’s length equals the run in the perpendicular direction.
Does this include top and bottom mat?
This tool estimates one mat. For double mats, run the calculation twice or double the result if both mats use the same layout.
How accurate is the weight estimate?
Weight uses industry-standard pounds per foot by bar size. Mill coatings, couplers, and chairs are not included—add a waste percentage for cuts and scrap.
Can I use this for a rebar grid in a footing?
Yes. Choose Footing or Grid, enter run length and footing width, and set spacing from your detail. Verify longitudinal vs transverse bar sizes on the plan if they differ.
What is the difference between rebar calculator and rebar estimator?
Searchers use both terms for the same workflow: quantity, length, and weight before ordering. This page combines spacing math with a live grid preview for field clarity.