Slab reinforcement calculator guide
The calculator above turns one rectangular slab into a complete reinforcement takeoff in two modes you switch between: a rebar grid — bar counts each way, running length, weight, plus support chairs and tie wire — or welded wire mesh — sheets across and down, purchased area and weight, plus the same chairs and ties. Everything runs in your browser, in metric or imperial, with a live plan-view diagram and an optional cost.
The guide below explains how the bar count comes from the clear span and spacing, how mesh sheets are tiled with overlap and rounded up to whole units, how chairs and tie wire are estimated as rules of thumb, and worked examples you can reproduce by hand — followed by reference tables for bar weight and mesh designations.
Rebar grid takeoff
A slab mat is two sets of bars at right angles, each set spaced across the perpendicular span inside the concrete cover. For each direction, the number of bars is the clear span divided by the spacing, rounded down, plus one for the bar that starts the run: bars = ⌊(span − 2 × cover) ÷ spacing⌋ + 1.
The two sets are counted separately — bars spaced across the width run the length direction, and bars spaced along the length run the width direction — then added for the total. Multiplying each set's bar count by its bar length gives the running length, and multiplying that by the bar's mass per unit length gives the weight.
Estimate a rebar grid in six steps
- Choose unitsPick metric (m, mm, kg) or imperial (ft, in, lb); every value converts when you switch.
- Enter the slab sizePick a slab-type preset or type the slab length and width; the preset also seeds a starting bar size, spacing and cover.
- Set the bar and spacingChoose the bar size and the on-center spacing — the same both ways, or a different spacing per axis.
- Set cover and stock lengthEnter the concrete cover and the stock bar length; a splice-lap allowance is added only when a bar runs longer than one stock length.
- Add chairs and tie wireAdjust the chair grid and the wire per tie if you want the support and tie-wire estimate, and add prices for an optional cost.
- Read the results, export or printCheck the bar count each way, running length, stock pieces and weight, then export to CSV or print the takeoff.
The rebar grid results come from these formulas, computed on canonical metres and converted for display.
Bars each way
bars = ⌊ (span − 2 × cover) ÷ spacing ⌋ + 1- Applied once per direction: the span is the perpendicular slab dimension, the cover is subtracted from both edges, and the +1 is the bar that starts the run.
- A non-positive clear span or spacing gives zero bars — the calculator never returns a negative or NaN count.
Running length & weight
length = Σ (bars × bar length); weight = length × mass per length- Each direction's bars are as long as the perpendicular clear span, so the two sets are summed separately.
- A flat splice-lap allowance (8% by default) is added only to a run whose bar is longer than the stock length; stock pieces round up from the lapped length.
A 6 × 4 m slab with a 12 mm bar at 200 mm each way, drawn by the same engine the live diagram uses. Bars are set out inside the cover inset.
Wire mesh layout
Welded wire mesh comes in fixed sheets (BS 4483 A-series) or rolls and sheets (ASTM A1064), and adjacent sheets must overlap at the seams. The number of sheets along an axis is driven by the effective step — the sheet dimension minus the overlap — not the raw sheet size, so a bigger overlap means more sheets.
Per axis: one sheet if the slab fits within a single sheet, otherwise ⌈(slab − sheet) ÷ (sheet − overlap)⌉ + 1. Multiply the two axis counts for the tiled sheets, add an edge/offcut waste allowance, and round up to whole purchasable units. Purchased area × the mesh mass per square metre gives the weight.
Estimate a wire mesh layout in six steps
- Choose unitsMetric shows the BS 4483 A-series; imperial shows the ASTM A1064 styles. Switch at any time and values convert.
- Enter the slab sizePick a preset or type the slab length and width; the diagram tiles the sheets as you type.
- Pick the mesh designationChoose the mesh style — it fixes the wire size, the sheet size and the mass per square metre.
- Set overlap and coverEnter the sheet overlap at the seams and the concrete cover; a bigger overlap means more sheets.
- Set the waste allowanceAdjust the edge/offcut waste percentage so the sheets round up to a realistic order.
- Read the results, export or printCheck the sheet grid, sheets to buy, purchased area and weight, then export to CSV or print the takeoff.
The wire mesh results come from these formulas, applied per axis so the layout never assumes a square sheet.
Sheets across & down
count = slab ≤ sheet ? 1 : ⌈ (slab − sheet) ÷ (sheet − overlap) ⌉ + 1- The effective step is the sheet dimension minus the seam overlap, so a larger overlap adds sheets.
- The two axis counts multiply for the tiled sheets before any waste.
Sheets to buy & weight
buy = ⌈ tiled × (1 + waste) ⌉; weight = buy × sheet area × mass per m²- Edge and offcut waste (10% by default) is added before rounding up to whole sheets or rolls.
- BS 4483 sheet mass is derived from the wire size and pitch; ASTM styles use the shipped weight per 100 ft².
An 8 × 6 m slab tiled with A252 sheets at a 200 mm overlap; the hatched bands are the laps where sheets overlap.
Support chairs and tie wire
Chairs hold the mat at the right height in the pour, and tie wire fixes the bars or mesh where they cross. Both quantities are rules of thumb here, not a code requirement — adjust them to your job and your inspector's expectations.
Chairs are laid on a grid (about one every metre / three feet each way by default) over the covered area, and tie wire is a length per tie times the number of tie points — every bar crossing for a mat, or every chair for mesh — at whichever frequency you choose.
Chairs and tie wire are rule-of-thumb quantities from these formulas — adjust them to your job.
Support chairs
chairs = (⌊ clear width ÷ spacing ⌋ + 1) × (⌊ clear length ÷ spacing ⌋ + 1); bags = ⌈ chairs ÷ 100 ⌉- Chairs sit on a grid (default 1 m / 3 ft each way) over the covered area, the same for both reinforcement modes.
- Chairs are sold in bags of 100 here — a rule of thumb, not a code requirement.
Tie wire
ties = tie points × frequency; wire = ties × length per tie; rolls = ⌈ wire ÷ roll length ⌉- Tie points are every bar crossing for a rebar mat, or every chair for mesh, at 'every' (×1) or 'every other' (×0.5).
- A roll is about 100 m (≈330 ft); the length per tie defaults to 200 mm / 8 in.
Cover and chairs in section
Cut through the slab and the reinforcement sits on chairs a fixed distance up from the bottom face — that gap is the concrete cover, and it protects the steel and gives the slab its strength. The cover also shortens the bars and narrows the grid, which is why the calculator subtracts it twice from each span before counting bars.
The section below shows a bar resting on chairs with the bottom cover called out. Cover, chair height and spacing all come from your drawings or the governing code — the calculator only counts quantities from the figures you enter.
Bar size and weight reference
The weight of a rebar mat is its running length times the bar's nominal mass per unit length. The tables below list the standard metric (BS 4449 / ISO 6935-2) and imperial (ASTM A615) bar sizes the calculator uses, with nominal diameter and mass — the same figures behind the weight result.
Nominal bar diameters and mass per unit length — the figures behind the weight result.
| Diameter | Mass |
|---|---|
| 6 mm | 0.222 kg/m |
| 8 mm | 0.395 kg/m |
| 10 mm | 0.616 kg/m |
| 12 mm | 0.888 kg/m |
| 16 mm | 1.579 kg/m |
| 20 mm | 2.466 kg/m |
| 25 mm | 3.854 kg/m |
| 32 mm | 6.313 kg/m |
| 40 mm | 9.864 kg/m |
| 50 mm | 15.413 kg/m |
| Bar | Diameter | Mass |
|---|---|---|
| #3 | 0.375 in | 0.376 lb/ft |
| #4 | 0.500 in | 0.668 lb/ft |
| #5 | 0.625 in | 1.043 lb/ft |
| #6 | 0.750 in | 1.502 lb/ft |
| #7 | 0.875 in | 2.044 lb/ft |
| #8 | 1.000 in | 2.670 lb/ft |
| #9 | 1.128 in | 3.400 lb/ft |
| #10 | 1.270 in | 4.303 lb/ft |
| #11 | 1.410 in | 5.313 lb/ft |
| #14 | 1.693 in | 7.650 lb/ft |
| #18 | 2.257 in | 13.600 lb/ft |
Nominal masses; actual delivered weight varies slightly by mill and standard.
Wire mesh designations
Mesh is specified by a designation that fixes the wire size, the spacing and therefore the mass per square metre. The table below lists the designations the calculator offers — BS 4483 A-series square sheets and the common ASTM A1064 styles — with wire size, spacing, mass, sheet size and how each is sold.
The mesh designations the calculator offers, with wire size, spacing, mass, sheet size and how each is sold.
| Designation | Standard | Wire | Spacing | Mass | Sheet size | Sold as |
|---|---|---|---|---|---|---|
| A98 | BS 4483 | 5 mm | 200 mm | 1.54 kg/m² | 2.4 × 4.8 m | Sheet |
| A142 | BS 4483 | 6 mm | 200 mm | 2.22 kg/m² | 2.4 × 4.8 m | Sheet |
| A193 | BS 4483 | 7 mm | 200 mm | 3.02 kg/m² | 2.4 × 4.8 m | Sheet |
| A252 | BS 4483 | 8 mm | 200 mm | 3.95 kg/m² | 2.4 × 4.8 m | Sheet |
| A393 | BS 4483 | 10 mm | 200 mm | 6.17 kg/m² | 2.4 × 4.8 m | Sheet |
| 6×6 – W1.4×W1.4 | ASTM A1064 | 0.014 in² | 6 in | 1.03 kg/m² | 5 × 150 ft | Roll |
| 6×6 – W2.9×W2.9 | ASTM A1064 | 0.029 in² | 6 in | 2.05 kg/m² | 5 × 10 ft | Sheet |
| 6×6 – W4.0×W4.0 | ASTM A1064 | 0.04 in² | 6 in | 2.83 kg/m² | 5 × 10 ft | Sheet |
| 4×4 – W1.4×W1.4 | ASTM A1064 | 0.014 in² | 4 in | 1.56 kg/m² | 5 × 10 ft | Sheet |
BS masses are derived from the wire size and pitch; ASTM masses come from the shipped weight per 100 ft². Confirm against your supplier before ordering.
Worked examples
Each example reproduces what the calculator returns so you can check the method by hand, using the tool's defaults — 50 mm / 2 in cover, 8% splice lap when bars exceed the stock length, 10% mesh waste, chairs on a 1 m / 3 ft grid, 200 mm / 8 in of wire per tie — unless stated.
Each example reproduces the calculator's output for the given inputs.
Rebar mat for a 6 m × 4 m slab
Metric · rebar grid- Given
- Slab 6 m long × 4 m wide
- 12 mm bar at 200 mm each way, 50 mm cover
- 12 m stock, chairs on a 1 m grid
- Formula
bars = ⌊ (span − 2 × cover) ÷ spacing ⌋ + 1- Steps
- Across the width: ⌊ (4 − 0.1) ÷ 0.2 ⌋ + 1 = ⌊ 19.5 ⌋ + 1 = 20 bars, each ≈ 5.9 m long
- Along the length: ⌊ (6 − 0.1) ÷ 0.2 ⌋ + 1 = ⌊ 29.5 ⌋ + 1 = 30 bars, each ≈ 3.9 m long
- Running length: 20 × 5.9 + 30 × 3.9 = 118 + 117 = 235 m (no splice — bars are under 12 m)
- Weight: 235 × 0.888 kg/m ≈ 209 kg
- Chairs: (⌊ 3.9 ⌋ + 1) × (⌊ 5.9 ⌋ + 1) = 4 × 6 = 24 chairs (1 bag)
- Result
- 50 bars, about 235 m of 12 mm, roughly 209 kg, plus 24 chairs and 2 rolls of tie wire
Rebar mat for a 20 × 20 ft slab
Imperial · rebar grid- Given
- Slab 20 ft × 20 ft
- #4 bar at 12 in each way, 2 in cover
- 40 ft stock
- Formula
bars = ⌊ (span − 2 × cover) ÷ spacing ⌋ + 1- Steps
- Clear span each way: 20 − 2 × (2 ÷ 12) ≈ 19.67 ft
- Bars each way: ⌊ 19.67 ÷ 1 ⌋ + 1 = 20, so 40 bars total, each ≈ 19.67 ft long
- Running length: 40 × 19.67 ≈ 787 ft (no splice — bars are under 40 ft)
- Weight: 787 × 0.668 lb/ft ≈ 525 lb
- Stock pieces: two 19.67 ft bars per 40 ft length ⌊ 40 ÷ 19.67 ⌋ = 2, so 40 ÷ 2 = 20 lengths
- Result
- 40 bars, about 787 ft of #4, roughly 525 lb, in 20 stock lengths of 40 ft
Wire mesh for an 8 m × 6 m slab
Metric · A252 mesh- Given
- Slab 8 m long × 6 m wide
- A252 sheet 2.4 × 4.8 m, 200 mm overlap
- 10% edge/offcut waste
- Formula
count = ⌈ (slab − sheet) ÷ (sheet − overlap) ⌉ + 1- Steps
- Across the 6 m width (2.4 m sheet): ⌈ (6 − 2.4) ÷ 2.2 ⌉ + 1 = ⌈ 1.64 ⌉ + 1 = 3
- Down the 8 m length (4.8 m sheet): ⌈ (8 − 4.8) ÷ 4.6 ⌉ + 1 = ⌈ 0.70 ⌉ + 1 = 2
- Tiled sheets: 3 × 2 = 6; with 10% waste ⌈ 6 × 1.1 ⌉ = 7 sheets
- Purchased area: 7 × 11.52 m² = 80.6 m²
- Weight: 80.6 × 3.95 kg/m² ≈ 318 kg
- Result
- A 3 × 2 grid, 7 A252 sheets to buy, about 81 m² and roughly 318 kg
Common mistakes to avoid
Forgetting the cover. The grid is set out inside the cover, so a bar count that uses the full slab span over-counts. Always subtract the cover from both edges of each span before dividing by the spacing.
Skipping the +1 bar. The clear span divided by spacing is the number of gaps, not bars — there is always one more bar than gaps. That single bar is the most common off-by-one error in a hand takeoff.
Ignoring mesh overlap. Sheets have to lap at the seams, so the usable step is the sheet minus the overlap. Tiling by the raw sheet size under-orders mesh on anything bigger than one sheet.
Treating chairs and ties as exact. These are rules of thumb, not a specification — increase them for a heavy mat or a fussy inspection, and never present them as a code figure.
Using the takeoff as a design. Bar size, spacing, cover and lap length must come from an engineer or the code. This tool counts quantities; it does not size reinforcement.
Frequently asked questions
Short answers to the questions people most often ask about estimating slab rebar and wire mesh with this calculator.
How much rebar do I need for a slab?
Count the bars each way, then turn them into length and weight. For each direction, bars = ⌊(span − 2 × cover) ÷ spacing⌋ + 1; multiply each set's bar count by its length for the running length, and by the bar's mass per unit length for the weight. A 6 × 4 m slab with 12 mm bars at 200 mm each way needs about 50 bars, 235 m and roughly 209 kg. The calculator does every step as you type.
Should I use a rebar grid or wire mesh?
Both reinforce a slab; the choice is usually availability, spec and labour. Loose bars tied into a mat suit heavier or engineered slabs and let you set the size and spacing exactly; welded wire mesh sheets go down faster for lighter slabs like paths and shed bases. The calculator computes both and shows the other option as a one-line alternative so you can compare quantity and weight.
How do I calculate the number of bars?
Subtract the cover from both edges of the span to get the clear span, divide by the on-center spacing, round down, and add one: bars = ⌊(span − 2 × cover) ÷ spacing⌋ + 1. The +1 matters — the division gives the number of gaps, and there is always one more bar than gaps. Do it once for each direction and add the two.
How many sheets of wire mesh do I need?
Tile the slab with sheets that overlap at the seams. Per axis, count = ⌈(slab − sheet) ÷ (sheet − overlap)⌉ + 1 (one sheet if the slab fits within a single sheet); multiply the two axes for the tiled sheets, add a waste allowance, and round up to whole sheets or rolls. The calculator does this with the sheet size for the designation you pick.
How much should wire mesh overlap?
A common rule is to lap sheets by at least one full mesh square, and not less than about 150 mm (6 in). The calculator defaults the overlap to the larger of the mesh pitch and 150 mm and lets you edit it. A bigger overlap uses a bit more mesh because it shortens the effective step between sheets.
How many chairs do I need?
Chairs go on a grid over the covered area — about one every metre or three feet each way is a common starting point. The calculator counts chairs = (⌊clear width ÷ spacing⌋ + 1) × (⌊clear length ÷ spacing⌋ + 1) and rounds up to bags of 100. This is a rule of thumb, not a code requirement, so adjust the spacing to your mat and inspection.
How much tie wire do I need?
Tie wire is a length per tie times the number of tie points. For a rebar mat the tie points are the bar crossings; for mesh they're the chairs. At about 200 mm (8 in) of wire per tie and one roll of roughly 100 m (330 ft), the calculator gives a tie count and roll count. Like chairs, it's a rule-of-thumb estimate you can adjust.
How much does slab rebar weigh?
Weight is the running length times the bar's nominal mass per unit length — for example 0.888 kg/m for a 12 mm bar or 0.668 lb/ft for a #4. The calculator sums the length of every bar in the mat (with a splice-lap allowance when bars exceed the stock length) and multiplies by the mass for your bar size to give kilograms or pounds.
How much concrete cover does a slab need?
Cover protects the steel and is set by your drawings or the code — commonly around 50 mm (2 in) to the bottom of a ground slab, more against earth or in aggressive conditions. The calculator uses the cover you enter to inset the grid and shorten the bars; it does not choose the cover for you, so take it from your specification.
Do I need lap splices, and how long are they?
You need a splice whenever a bar runs longer than the stock length you can buy. The calculator adds a flat splice-lap allowance (8% by default) to any run that exceeds the stock length, which is enough for a takeoff. For the exact lap length for a given bar and concrete, use the lap and development length calculator linked in the related tools.
Does it work in metric and imperial?
Yes. Enter the slab in metres or feet and cover and spacing in millimetres or inches, with weight in kilograms or pounds. Metric shows BS 4449 bars and BS 4483 mesh; imperial shows ASTM bars and mesh. Switch the unit toggle at any time and every input and result converts.
Can I export or print the result?
Yes. Export CSV downloads the takeoff for Excel, Google Sheets or Numbers, and Print produces a clean sheet with the bar or mesh quantities, weight, chairs, ties and any costs — handy to take to the supplier.
Is the calculator private?
Yes. Every calculation runs in your browser — nothing you enter is uploaded to a server. Your project is saved only in your own browser's local storage, so you can close the tab and return to it later.