How much overlay do you need for an existing slab?
You need enough overlay material to cover the existing slab footprint at the thickness the product data sheet states, once per coat. The calculator multiplies area by thickness per coat and by the coat count, and returns cubic feet and cubic yards in imperial mode, or cubic metres and litres in metric mode.
An overlay is not a new slab. The existing slab stays in place and carries the load, so the quantity task is different: the layer is thin, the thickness comes from the product, and a small measuring error is a large share of the total. Keep the measured volume visible before any allowance or bag conversion.
Record measured volume first, planning volume second, and whole bags last. Each stage has a different source and can change for a different reason.
| Result | Calculation | What you must confirm |
|---|---|---|
| Measured volume | Area × thickness per coat × coats | Footprint, thickness, coat count, and units |
| Planning volume | Measured volume × (1 + entered allowance ÷ 100) | Reason and evidence for the allowance |
| Whole bags | Planning volume ÷ entered label yield, rounded up | Exact product, yield unit, and bag size |
| Coverage per bag | Label yield ÷ thickness per coat | Agreement with the data-sheet coverage at that thickness |
When should you use this overlay calculator?
Use it for one existing slab footprint covered at one thickness per coat. Run separate calculations when the footprint, thickness, product, or coat count changes.
| Project condition | Calculator method | Record to keep |
|---|---|---|
| Sound existing slab, one uniform area | Enter footprint, thickness, and coats | Slab condition check and data-sheet thickness |
| Round patio or pad | Enter the full diameter and thickness | Diameter measured through the centre |
| Area already known | Use known-area mode and enter thickness | Drawing, survey, or separate area calculation |
| Openings, drains, or planters in the slab | Deduct each opening area before entering the footprint | Opening dimensions and deduction record |
| L-shaped or irregular footprint | Divide it into simple sections and add unrounded volumes | Sketch and section identifiers |
| Low spots or pitted areas | Calculate low spots as separate sections at their own extra thickness | Depth readings and section limits |
| New slab with no existing concrete | Use the Concrete Slab Calculator instead | New-slab dimensions and edge details |
Check the slab before measuring. Deep, wide, or actively spreading cracks, especially with one side higher than the other, signal settlement or subgrade instability. An overlay does not repair a moving slab, and the quantity math cannot certify that the slab suits one. The concrete overlay guide covers the product and condition checks that come before this calculation.
Which measurements do you need?
You need the existing slab footprint and the overlay thickness per coat. Measure in the units printed beside the fields and record where each value came from.
Footprint
Measure the usable limits of the existing slab. Edging, kerbs, structures, drains, islands, and openings change the plan area. Split an abrupt shape change into a separate section.
Thickness per coat
Read the thickness from the product data sheet for the exact product you will buy. For QUIKRETE Re-Cap Concrete Resurfacer #1131-47, the stated range runs from a feather edge of 1/16 in to 1/2 in. Other products state different ranges, and no range transfers to another brand. The calculator never selects a thickness.
Coats
Enter 1 unless the data sheet calls for a two-coat procedure. QUIKRETE Concrete Resurfacer #1131-40 lists a two-coat procedure as optional but recommended. Each coat is entered at the same per-coat thickness, so 2 coats double the measured volume.
Allowance and label yield
The allowance field starts at 0%. Use a value only when surface condition, placement, handling, or a project requirement supports it. The label yield is the volume printed for the exact bag you will buy. A yield from another product, bag size, or material condition does not belong in this field.
What formulas does the calculator use?
The calculator finds plan area, converts the per-coat thickness to the base length unit, multiplies by the coat count, and keeps full JavaScript precision during the calculation. Displayed results show up to 3 decimal places.
Rectangle area = length × width
Circle area = π × (diameter ÷ 2)²
Measured volume = area × thickness per coat × coats
Planning volume = measured volume × (1 + allowance % ÷ 100)
Whole bags = planning volume ÷ label yield, rounded up
Coverage per bag = label yield ÷ thickness per coat
| Displayed conversion | Relationship | Calculator use |
|---|---|---|
| Inches to feet | 12 in = 1 ft | Imperial thickness conversion |
| Millimetres to metres | 1,000 mm = 1 m | Metric thickness conversion |
| Cubic feet to cubic yards | 27 ft³ = 1 yd³ | Imperial measured equivalent |
| Cubic metres to litres | 1 m³ = 1,000 L | Metric measured equivalent |
NIST Handbook 44 Appendix C supplies the cited volume relationships. Unit switching converts populated dimensions, thickness, and yield, clears the displayed result, and requires a fresh calculation in the selected units.
Worked imperial example
A rectangular patio is 20 ft long, 12 ft wide, and covered at 1/4 in in 1 coat, with a 10% allowance and a label yield of 0.5 ft³ per bag.
- Area: 20 × 12 = 240 ft².
- Thickness in feet: 0.25 ÷ 12 = 0.0208333 ft.
- Measured volume: 240 × 0.0208333 = 5 ft³.
- Measured equivalent: 5 ÷ 27 = 0.185 yd³.
- Planning volume: 5 × 1.10 = 5.5 ft³.
- Whole bags: 5.5 ÷ 0.5 = 11 bags.
- Coverage per bag: 0.5 ÷ 0.0208333 = 24 ft².
Measured result: 5 ft³ and 0.185 yd³. The planning volume is 5.5 ft³, and the order is 11 whole bags. The coverage figure is a cross-check, not a second order quantity.
Worked metric example
A circular pad is 6 m in diameter, covered at 6 mm per coat in 2 coats, with no allowance and a label yield of 0.016 m³ per bag.
- Area: π × (6 ÷ 2)² = 28.274 m².
- Thickness in metres: 6 ÷ 1,000 = 0.006 m.
- Volume per coat: 28.274 × 0.006 = 0.170 m³.
- Measured volume: 0.170 × 2 = 0.339 m³.
- Measured equivalent: 0.339 × 1,000 = 339 L.
- Whole bags: 0.339 ÷ 0.016 = 21.21, rounded up to 22 bags.
- Coverage per bag: 0.016 ÷ 0.006 = 2.667 m².
Measured result: 0.339 m³ and 339 L. The order is 22 whole bags. The second coat doubles the volume because each coat covers the same area at the same thickness.
How do you check a label yield against stated coverage?
Some overlay products publish coverage per bag instead of a volume yield. The two figures describe the same bag at the same thickness, so they must agree. Coverage per bag equals yield divided by thickness, and yield equals coverage multiplied by thickness in matching units.
Example: QUIKRETE Re-Cap covers approximately 17 ft² per 40 lb bag at 1/4 in. The implied yield is 17 × 0.25 ÷ 12 = 0.354 ft³ per bag. Enter 0.354 ft³ as the label yield and the calculator returns 17 ft² per bag, which matches the published coverage. If the calculator returns a very different coverage figure, the yield, thickness, or product does not match the published figure, and the mismatch must be resolved before ordering.
Published coverages are approximate and vary with thickness, temperature, substrate, and application method. A pitted or spalled surface consumes more material than the nominal figure, which is why the allowance and the low-spot sections stay separate from the measured volume.
Why does a small thickness error matter so much on a thin layer?
Thickness multiplies directly into volume, and the layer is thin, so a small absolute error is a large share of the total. A 100 ft² area at 1/16 in needs 0.521 ft³. The same area at 1/2 in needs 4.167 ft³, which is 8 times as much. A 1/16 in measuring error on a 1/4 in layer is a 25% quantity error.
Take thickness readings across the whole slab, not at one convenient point. Measure from the highest point of the existing surface, because the finished layer must cover the high spots at the stated thickness, and every low spot adds material on top of the nominal layer. Recalculate from the changed measurement instead of adding an unexplained percentage.
Common overlay calculation mistakes
- Entering a new-slab thickness, such as 4 in, for a thin overlay product.
- Using one thickness from a different product or brand.
- Forgetting the second coat when the data sheet calls for a two-coat procedure.
- Measuring the overlay from the low spots instead of the highest point of the old surface.
- Applying a slab waste percentage without a project reason.
- Using a poured-concrete bag yield for a thin overlay product.
- Entering radius in the diameter field, which cuts a circular area to one quarter.
- Rounding every section before adding the total.
- Treating a calculated bag count as proof that the existing slab suits an overlay.
- Ordering from a coverage figure at one thickness while entering a different thickness.
Questions about concrete overlay quantity
Can this calculator choose the overlay thickness?
It uses the thickness you enter. The product data sheet, substrate condition, finish, and responsible project people determine the thickness. A thickness from another product does not transfer.
Does the calculator check whether the slab suits an overlay?
It calculates quantity only. Slab soundness, crack movement, bond, preparation, and structural capacity are separate checks that belong to the project record and a qualified person.
Should the coat count always be 2?
Use 2 only when the data sheet for the exact product calls for a two-coat procedure. The default is 1, and each coat adds its full volume at the entered thickness.
Does the allowance field cover surface repairs?
It applies one user-entered percentage to measured volume. Repair quantities, primer, and low-spot fill are separate records with their own measurements.
Can the bag count set the order on its own?
The bag count covers the planning volume at the entered yield. Selling unit, minimum order, delivery increment, and partial-bag rules come from the current supplier quote.
Does the calculator estimate overlay cost?
It calculates quantity. Cost needs a current quote for the exact product, bag size, amount, delivery address, taxes, access conditions, fees, and quote validity period.
Sources and scope
- NIST Handbook 44 (2026), Appendix C: foot, inch, cubic-foot, cubic-yard, and cubic-metre relationships; accessed 28 July 2026.
- QUIKRETE, Concrete Resurfacer #1131-40 product data: approximately 17 ft² per 40 lb bag at 1/4 in, approximately 90 ft² at minimum thickness with broom or squeegee, optional but recommended two-coat procedure; accessed 14 October 2026.
- QUIKRETE, Re-Cap Concrete Resurfacer #1131-47 specification sheet: feather edge 1/16 in to 1/2 in application thickness; approximately 17 ft² per 40 lb bag at 1/4 in or approximately 65 ft² at 1/16 in with broom or squeegee; accessed 14 October 2026.
- Consolidated Aggregate Products, Concrete Overlays Reference Guide: per-50-lb-bag coverage charts at stated thicknesses, described as approximate and varying with thickness, temperature, and conditions; accessed 14 October 2026.
Source scope: NIST supports the stated conversions. QUIKRETE and Consolidated Aggregate Products support the named-product thickness ranges and coverage figures, which do not transfer to other products. Project drawings, specifications, surveys, supplier records, product data sheets, substrate checks, local rules, and responsible people control the product, thickness, coat count, order, and work plan.
Review note: Calculation logic and independent automated fixtures passed internal review. Saleem Sial is the research and editorial owner. Waseem Sial's external review remains ongoing. The published page is indexable.
