Rebar Calculator
Estimate rebar pieces, linear feet, weight, and project cost. Calculate grid layouts, lap splices, chairs, and tie wire for slabs, footings, foundation walls, and columns.
Example Projects
Concrete Patio Slab
15' × 12' slab · #3 rebar @ 12" O.C.
Residential Driveway
40' × 20' slab · #4 rebar @ 18" O.C.
House Foundation Slab
60' × 40' slab · #5 rebar @ 16" O.C. (Double Mat)
Foundation Trench Footing
100' long · 3 long bars (#4) + transverse @ 12"
Deck Footing Beam
50' long · 2 long bars (#3) · No ties
Retaining Wall
50' × 6' wall · #5 rebar @ 12" vert & horiz
Basement Wall
120' × 8' wall · #4 rebar @ 16"/18" (Double Mat)
Round Support Pier
12" Dia × 10' high · 4 vert (#4) + #3 ties @ 8"
Structural Column
18"×18" × 15' high · 8 vert (#6) + #3 ties @ 6"
Custom Rebar Mat
20' × 20' · Custom grid: 15 rows × 15 columns
Project Settings View Results
Rebar Estimate
View Calculation Details
| Layout Stats | 0 rows × 0 columns (0 intersections) |
| Length-direction Pieces | 0 |
| Width-direction Pieces | 0 |
Grid Calculations Breakdown:
Everything behind the Rebar Calculator
Formulas, reference charts, and detailed answers — expand any section you need.
Rebar Size Chart: #3 to #8 and Metric Equivalents
Rebar size is identified by a number corresponding to its diameter in eighths of an inch. A #4 bar is ½" in diameter; a #5 bar is ⅝". Metric bar sizes (10M–20M) are commonly cross-referenced on Canadian and international projects.
| Bar Size | Diameter | Weight per Foot | Metric Equivalent |
|---|---|---|---|
| #3 | 0.375 in | 0.376 lb/ft | 10M |
| #4 | 0.500 in | 0.668 lb/ft | 10M–15M |
| #5 | 0.625 in | 1.043 lb/ft | 15M |
| #6 | 0.750 in | 1.502 lb/ft | 20M |
| #7 | 0.875 in | 2.044 lb/ft | 20M–25M |
| #8 | 1.000 in | 2.670 lb/ft | 25M |
Rebar Spacing Guide by Application
Spacing requirements vary by structural application and local building code. These are common industry defaults used for estimating — actual spacing should always follow a stamped engineering drawing where one exists.
| Application | Typical Spacing |
|---|---|
| Residential slab-on-grade | 16"–18" O.C. |
| Driveway/sidewalk | 18"–24" O.C. |
| Footings | 12"–16" O.C. |
| Foundation walls | 12"–18" O.C., double mat |
Rebar Calculator: How Much Rebar Do I Need for a Slab?
Rebar quantity for a slab is driven by three variables: the slab's dimensions, the on-center spacing between bars, and whether the design calls for a single or double mat. Divide the slab length by the spacing to get the number of bars running one direction, then divide the width by the spacing for bars running the other direction. Multiply each count by the length of the perpendicular dimension to get linear footage, then add lap splice length for any run exceeding standard bar stock (20ft or 40ft).
Example: A 20ft × 20ft slab with #4 rebar at 18" on-center needs 14 bars each direction — 28 bars total, roughly 560 linear feet before splices and waste.
Rebar Lap Splice Length Calculator
Since rebar is sold in fixed stock lengths, any run longer than the stock length requires a lap splice — overlapping two bars so they act as one continuous piece. Standard splice length is typically 24 to 40 times the bar diameter, depending on concrete strength and bar grade. Skipping or under-sizing the splice is a common source of structural failure in DIY slab and footing pours.
Rebar Weight Calculator: Converting Linear Feet to Tons
Rebar is often priced and delivered by the ton rather than by the piece. To convert linear footage into weight, multiply total linear feet by the weight-per-foot value for the chosen bar size. This is especially useful for large foundation or commercial slab pours where suppliers quote pricing per ton rather than per stick.