ganak.appRebar Calculator
Free rebar calculator. Estimate rebar quantity, spacing, weight, and cost for concrete slabs, footings, and walls. Instant results with bar size reference.
Standard: 20 ft or 60 ft
20 × 20 ft · 12 in o.c.
Rebar Summary
Quantity, length, weight, and material estimates
Calculation Breakdown
Step-by-step rebar grid calculation
| Item | Value |
|---|---|
| Slab / Pad dimensions | 20 × 20 ft |
| Edge clearance | 3 in per side |
| Grid (after clearance) | 19.5 × 19.5 ft |
| Spacing | 12 in o.c. |
| Lengthwise bars | 20 × 19.5 ft |
| Widthwise bars | 20 × 19.5 ft |
| Total linear length | 780 ft |
| Total | 40 bars · 780 ft |
Rebar Size Reference
US standard rebar sizes (ASTM A615)
| Size | Dia (in) | lb/ft | kg/m |
|---|---|---|---|
| #3 | 0.375 | 0.376 | 0.56 |
| #4 | 0.5 | 0.668 | 0.994 |
| #5 | 0.625 | 1.043 | 1.552 |
| #6 | 0.75 | 1.502 | 2.235 |
| #7 | 0.875 | 2.044 | 3.042 |
| #8 | 1 | 2.67 | 3.973 |
| #9 | 1.128 | 3.4 | 5.06 |
| #10 | 1.27 | 4.303 | 6.404 |
What Is a Rebar Calculator?
Overview of the tool and what it helps you estimate.
A rebar calculator estimates how much steel reinforcing bar (rebar) you need for a concrete slab, footing, or wall. It calculates the number of bars, total linear length, weight, and stock bars to purchase based on your slab dimensions, bar spacing, and edge clearance.
Rebar strengthens concrete by resisting tensile forces that concrete alone cannot handle. Without proper reinforcement, concrete slabs are prone to cracking under load, temperature changes, and settling.
#4
most common
Residential slabs
12"
typical spacing
On-center (o.c.)
3"
edge clearance
Standard cover
Rebar Calculation Formulas
Core formulas used for rebar grid estimation.
Grid dimensions (after edge clearance)
Bar count per direction
Total linear length
Weight calculation
Rebar Spacing Guide
Common spacing recommendations by application.
| Application | Bar Size | Spacing |
|---|---|---|
| Patio / sidewalk (4" slab) | #3 or #4 | 12-18" |
| Driveway (4-5" slab) | #4 | 12" |
| Garage floor (4-6" slab) | #4 | 12" |
| Foundation footing | #4 or #5 | 6-8" |
| Retaining wall | #5 or #6 | 8-12" |
| Structural slab | #5 or #6 | 6-12" |
Always follow your local building codes and engineer's specifications. These are general guidelines for typical residential construction.
Rebar Calculator Examples
Sample calculations for common projects.
20 x 20 ft driveway slab
#4 bar, 12" spacing, 3" clearance
10 x 10 ft patio
#4 bar, 12" spacing, 3" clearance
30 x 2 ft footing
#5 bar, 6" spacing, 3" clearance
24 x 8 ft retaining wall
#5 bar, 8" spacing, 3" clearance
Common Mistakes to Avoid
Pitfalls that lead to over- or under-ordering rebar.
Forgetting edge clearance
Rebar must be inset from slab edges by at least 3" (75 mm) to prevent corrosion. Without clearance, your bars are too long and too numerous.
Not accounting for overlap / splices
Standard rebar comes in 20 ft (6 m) lengths. Longer runs require splice overlaps of 24-40 bar diameters, adding 10-15% to total length.
Using the wrong bar size
#3 rebar is too thin for most structural work. #4 is the minimum for driveways and garage slabs. Footings often require #5 or larger.
Ignoring building codes
Spacing, bar size, and cover requirements vary by jurisdiction. Always check local codes or consult a structural engineer before pouring.
Ordering exact quantities
Always add 10% waste for cuts, bends, and field adjustments. Complex shapes with many cuts may need 15%.
Pro Tips
Expert advice for accurate rebar estimation.
Place rebar on chairs or supports to maintain proper cover depth — rebar sitting on the ground provides no structural benefit.
For slabs over 5" thick, consider placing rebar at the bottom third of the slab thickness for maximum tensile strength.
Tie wire at every other intersection for slabs. Tie every intersection for walls and structural elements.
In cold climates, increase edge clearance to 2" (50 mm) minimum to account for freeze-thaw damage to concrete cover.
Number 4 rebar (#4, 1/2") is the sweet spot for most residential work — strong enough for structural loads, easy to cut and bend by hand.
Frequently Asked Questions
Common questions about rebar calculation and reinforcement
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