Slab on Grade Calculator — Concrete, Mesh/Rebar & Vapor Barrier
Estimate materials for a concrete slab on grade. Enter one or more slab rectangles, thickness, and waste allowance. Choose reinforcement (wire mesh, rebar grid, or fibers) and plan a vapor barrier roll layout with overlaps. Get concrete volume (m³/yd³/ft³), bag counts, mesh/rebar/fiber quantities, vapor barrier rolls & tape length, and an auto-grouped rebar cut list.
Results
Concrete
Net area: 0.000 m²
Thickness: —
Net volume: 0.000 m³
With waste: 0.000 m³
0.000 yd³ • 0.00 ft³
Bag counts (from total with waste)
40 lb: 0 • 50 lb: 0
60 lb: 0 • 80 lb: 0
Reinforcement summary
—
Estimated steel weight shown for rebar mode.
Vapor barrier & insulation
Barrier rolls: 0
Approx. tape length: —
Insulation boards: —
Per-rectangle areas
| # | Length | Width | Area |
|---|
Reinforcement (details)
Vapor Barrier & Insulation
Barrier coverage
Roll effective width: —
Coverage per roll: —
Insulation
Board size: —
Total area: —
Control-Joint Suggestions
Rule-of-thumb spacing: —
Based on 24–36× slab thickness (in) for sawed joints; verify with your engineer and finish contractor.
Cut List (grouped rebar lengths)
| Dir | Bar size | Qty | Length | Note |
|---|
Cut list assumes stock-length repeats with a shorter “remainder” last piece; laps modeled as overlaps (extra steel).
How to use this slab calculator
- Select units, enter rectangles, thickness, and waste %.
- Pick reinforcement mode: mesh (sheet/roll count), rebar grid (spacing, bar size), or fibers (dosage).
- Enable vapor barrier and set roll size, lap, and waste; optionally include underslab insulation.
- Review volumes, bags, reinforcement, VB rolls & tape, joints, and (if rebar) the cut list.
Formulas & assumptions
- Area:
A = Σ(L × W)across rectangles. - Concrete volume:
V = A × t; with wasteVw = V × (1 + waste%). - Bag counts:
bags = ⌈Vw,ft³ / yieldper bag⌉(user-editable yields). - Mesh coverage: effective coverage per sheet/roll
=(W − lap) × (L − lap)→ count= ⌈ A / coverage ⌉(planning estimate). - Rebar grid (single mat): clear spans
Lclr=L−2×cover,Wclr=W−2×cover. Bars each way:n(L)=max(2, ⌊Wclr/sY⌋+1),n(W)=max(2, ⌊Lclr/sX⌋+1). Per-bar length:Lreq,X=Lclr,Lreq,Y=Wclr. IfLreq>Lstock, laps:Llap=max(lapDb×db, lapMin); extra steel per bar=(k−1)×Llapwith advanceLstock−Llap. - Fiber mass:
M = Vw,m³ × dosage (kg/m³). - Vapor barrier: effective roll width
= Wroll − lap; coverage/roll= effective width × roll length. Rolls= ⌈ A × (1+VB%)/coverage ⌉. Tape length is an approximation based on seam count. - Joint spacing: max panel dimension (ft) ≈ 24–36 × slab thickness (in). Use the lower end for crack control on interior slabs.
- Scope: Estimator only—does not design thickness, joints, subbase, curling, or load transfer. Follow plans/specs and local code.
References
- ACI 302.1R — Guide for Concrete Floor and Slab Construction
- ACI 360R — Design of Slabs-on-Ground
- ASTM E1745 — Standard Specification for Plastic Vapor Retarders Under Concrete Slabs
- CRSI — Reinforcing bar sizes, weights & properties
- ACI 224R (preview) — Joints in Concrete Construction
- QUIKRETE® Concrete Mix 1101 — typical bag yields
- ASTM A1064 — Standard Specification for Steel Wire/WWR for Concrete
FAQs
Mesh vs. rebar vs. fibers — which should I pick?
Mesh helps with crack control when properly chaired and tied. Rebar grids are stiffer and common for heavier loads. Fibers can reduce plastic shrinkage cracking; macro fibers can supplement or replace light mesh in some specs. Follow your engineer’s design.
Where should the vapor barrier go?
For moisture-sensitive floor coverings or slabs with low permeability requirements, use an ASTM E1745 Class A vapor retarder/barrier with sealed laps, typically directly under the slab unless the spec requires a blotter layer. See ACI 302.1R guidance.
How are joint spacings determined?
Rules of thumb (24–36× slab thickness in inches) are starting points only. Joint layout must consider aspect ratio, openings, re-entrant corners, columns, and curing; see ACI 302.1R and ACI 360R.