# Slab Reinforcement Calculator — Rebar Grid & Wire Mesh > Free slab reinforcement calculator: bar counts, running length, welded wire mesh sheets, weight, support chairs and tie wire for a concrete slab — with a live layout diagram and optional cost, all in your browser. Bar counts, mesh sheet layout, weight, chairs and tie wire for a concrete slab — rebar grid or welded wire mesh, entirely in your browser. - **HTML page:** https://siteometry.com/en/tools/slab-reinforcement-calculator - **Category:** [Concrete & Masonry](https://siteometry.com/en/tools/category/concrete-masonry) - **Figures live on the page:** tables, diagrams and worked figures the text refers to ("the table below") are drawn by the page and are not reproduced here. Each section below links to its anchor on the page. - **Runs in the browser:** Siteometry calculators need no account and send no input anywhere — every number below is computed on the visitor's own device. ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#guide-intro_ ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#rebar-takeoff_ ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#mesh-layout_ ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#chairs-ties_ ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#cross-section_ ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#bar-weights_ ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#mesh-table_ ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#examples_ ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#mistakes_ ## 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. _On the page: https://siteometry.com/en/tools/slab-reinforcement-calculator#faq_ ## Related calculators - [Steel Quantity Estimator — Steel per m³ of Concrete](https://siteometry.com/en/tools/steel-quantity-estimator) - [Sonotube & Pier Concrete Calculator](https://siteometry.com/en/tools/sonotube-calculator) - [Concrete Volume & Bag Calculator](https://siteometry.com/en/tools/concrete) - [Concrete Mix Ratio Calculator](https://siteometry.com/en/tools/concrete-mix-ratio-calculator)