A countertop order starts with the finished outline, not the cabinet box. Enter the finished length, finished depth, and as-built slab thickness, then add a backsplash, a thickened front edge, or a sink cutout only when the project has one. The calculator keeps each part as its own volume line, so a shortfall stays traceable to the right line.
How much concrete does a countertop run need?
Multiply finished length by finished depth by slab thickness, all in the same unit, then subtract every full-thickness cutout. A straight run of 72 in by 25.5 in at 1.5 in thick, with a 24 in by 18 in sink cutout, measures 1.22 ft3 net. At a label yield of 0.60 ft3 per bag, that run takes 3 whole bags. The worked example below shows each step.
How do you measure a countertop run?
Measure the finished countertop, not the cabinet it sits on. The measurement guide for concrete countertops records eight readings, and three of them drive this calculator directly.
| Reading | How to take it | What it fixes |
|---|---|---|
| Finished length | Along the finished edge at counter height, broken at seams and appliance gaps | Gross volume and seam positions |
| Finished depth | Back wall or cabinet back to the finished front edge, with overhangs included | Gross volume |
| Slab thickness | Inside the assembled form, at several points | Volume and mix choice |
| Backsplash | Length it actually runs, plus height and thickness, recorded separately | Backsplash volume |
| Sink cutout | Length and width from the manufacturer cutout template | Volume deduction |
Add every overhang before you multiply. A depth reading taken at the cabinet face under-reads the finished slab, because the front overhang is part of the cast concrete. Read the thickness inside the assembled form as well: a 0.25 in gap between the drawing and the form, over a 12 ft2 slab, adds 0.25 ft3, which is almost half a bag at a 0.60 ft3 yield.
What is the formula?
Top area = finished length x finished depth
Gross slab volume = top area x slab thickness
Backsplash volume = backsplash length x backsplash height x backsplash thickness
Additional edge volume = finished length x edge band depth x (total edge height - slab thickness)
Sink cutout volume = cutout length x cutout width x slab thickness
Measured net volume = gross slab volume + backsplash volume + additional edge volume - sink cutout volume
Planning volume = measured net volume x (1 + planning allowance / 100)
In imperial units, divide cubic inches by 1,728 to get cubic feet. The divisor is exact: 1 ft equals 12 in, so 1 ft3 equals 1,728 in3. One cubic yard equals 27 ft3, and one cubic metre equals 1,000 litres. The calculator shows the measured volume and the planning volume as separate results. It never merges them, and it never rounds an intermediate value: the bag division uses the unrounded planning volume.
Three volume states stay separate on this page. Measured volume is geometry only. Planning volume is measured volume plus an allowance the reader enters. Whole bags are a buy quantity from the reader's label yield, rounded up. Cast concrete has no loose volume, swell, shrinkage, or compaction stage, so none of those concepts appear in the calculation. Waste, handling loss, and contingency enter only through the reader's own allowance line.
Worked example: one straight run with a sink cutout
A straight run measures 72 in long and 25.5 in deep, at 1.5 in thick, with a 24 in by 18 in sink cutout. The same run appears in the worked countertop example.
| Line | Arithmetic | Result |
|---|---|---|
| Top area | 72 x 25.5 in | 12.75 ft2 |
| Gross slab volume | 72 x 25.5 x 1.5 / 1,728 | 1.59375 ft3 |
| Sink cutout volume | 24 x 18 x 1.5 / 1,728 | 0.375 ft3 |
| Measured net volume | 1.59375 - 0.375 | 1.21875 ft3, shown as 1.22 ft3 |
| Whole bags | 1.21875 / 0.60, rounded up | 3 whole bags |
The cutout removes more than half a bag at the stated yield. Skip that line and the order comes up short. A second check in metric units, with every optional part included, gives the same structure: a 240 cm by 65 cm run at 4 cm thick, with a 240 cm by 10 cm by 4 cm backsplash, a 5 cm edge band at 7.5 cm total height, and a 60 cm by 45 cm sink cutout, measures 0.0654 m3 net. At a 10% allowance the planning volume is 0.07194 m3, and at a 0.017 m3 label yield that is 5 whole bags. Both examples are covered by automated tests on the page.
How do backsplash and edge profiles change the volume?
A backsplash is a vertical strip cast with the top or fixed to it, so its volume is its own rectangle: length times height times thickness. Enter the length the backsplash actually runs. A backsplash that stops short of the full run must not be charged at the full run length.
A thickened front edge adds concrete only where the edge is deeper than the field. A 3 in tall edge on a 1.5 in slab adds a strip volume of length times band depth times 1.5 in, not a second full slab. The calculator asks for the total edge height and subtracts the slab thickness itself, so the field volume is never counted twice. The band depth must stay smaller than the finished depth, and the total edge height must exceed the slab thickness, or the calculator rejects the entry.
Countertop thickness commonly runs 1.5, 2, and 3 in for full-depth casts. A glass fiber reinforced thin shell at 3/4 to 1 in is a different casting method with its own mix design, described in the countertop methods guide. For quantity purposes it is still a thickness input on this page: enter the shell thickness and the same geometry gives the thinner volume. The mix, fiber, and polymer design behind that shell are outside this calculator.
How do sink cutouts change the volume?
Deduct every full-thickness cutout from the gross volume, using the manufacturer cutout template for length and width. The cutout length must stay smaller than the finished length, and the cutout width must stay smaller than the finished depth. A cutout that reaches an edge is not a cutout at all: it changes the outline, so the run must be re-measured as separate pieces.
Faucet holes are not deducted. Their volume is negligible at countertop scale, and the measurement guide treats faucet positions as form layout, not volume. If a project has several sinks, calculate each piece separately or add the cutouts on the order record: this calculator deducts one sink cutout per run, matching the single-cutout worked example.
How do allowance and bags work?
The planning allowance starts at 0. No universal allowance is published on this page, because label yields carry no allowance and the right allowance depends on the project: a test pour, a slow form leak, and a color-sample batch each eat into the total in ways one percentage cannot predict. Enter your own value and keep the reason on the order record. The countertop ordering guide records the allowance in bags, not just as a percentage, so the store trip and the pour day can both be checked.
Whole bags come from the label yield of the bag you will buy, entered in the calculator. The QUIKRETE Countertop Mix No. 1106-80 data sheet, also recorded in the countertop mix guide, states about 0.60 ft3 per 80 lb bag. That figure belongs to that product only. Other products state other yields, so the calculator accepts any yield you enter and shows no bag count when the field is empty. Bags always round up: a fraction of a bag cannot be bought, and rounding down is how a pour runs short. The bag count covers the planning volume, so an entered allowance is included in the buy count while the measured net volume stays visible beside it.
Buy all the bags in one trip when color matters, so the lot stays consistent. Bagged premix is usually a same-day pickup, but check store stock before pour day. For larger countertop volumes, the concrete bags calculator converts a known volume to bags across bag sizes.
Common mistakes
- Measuring depth at the cabinet face and leaving the front overhang out of the volume.
- Using the drawing thickness instead of the as-built thickness inside the assembled form.
- Leaving a thickened front edge out of the volume and running the front edge dry.
- Skipping the sink cutout deduction, which removes more than half a bag on the worked run.
- Ordering on unrounded volume instead of whole bags.
- Treating a fixed 10% waste figure as a rule, when the correct allowance depends on the project and the label yield carries none.
- Feeding an L-shaped or curved run into a single rectangle instead of calculating each straight piece separately.
Limits
This calculator covers one straight countertop run. L-shaped runs, curved runs, islands, and multi-piece runs are out of scope as a single calculation: split the run at seams and calculate each straight piece on its own, as the methods guide describes for runs over about 7 to 8 ft. The page does not design the countertop, choose the mix, set reinforcement, check cabinet support, or price the job. Countertop weight depends on the mix, so no weight output is given: a universal density would mislead. Check the project detail for thickness, edge profile, and cutouts, and check the bag label for the yield before ordering. Quantity planning does not replace structural design, project drawings, specifications, permits, or a qualified professional.
Sources and review status
- NIST Handbook 44 (2026), Appendix C: cubic-foot, cubic-yard, litre, and cubic-metre volume relationships used in the calculator.
- TCC Materials, Premium Concrete Countertop Mix installation guide: inch-based volume method (length times width times thickness, divided by 1,728), 1.5, 2, and 3 in casting thicknesses, and the no-allowance label-yield rule.
- QUIKRETE, Countertop Mix No. 1106-80 data sheet: one named product stating about 0.60 ft3 per 80 lb bag, used here only as an example of what a label yield looks like.
- Concrete Network countertop thickness guide: typical countertop thickness of 1.5 to 2 in, and mix-specific weight context.
Source scope: NIST supports the unit conversions. TCC Materials supports the volume method and the casting thicknesses. QUIKRETE supports the single named product yield used as an example. Concrete Network supports the thickness context. These sources do not set project dimensions, edge detail, allowance, a universal bag yield, mixture, reinforcement, price, or accepted order quantity. The site's published countertop measurement, mix, ordering, mistakes, example, and methods guides record the project workflow this calculator follows.
Return to the Concrete Planning hub for related quantity and supply tools. The calculation methodology explains the site's unit conversion, precision, rounding, assumptions, and correction process.
Review note: Saleem Sial owns the research and editorial record. Formula fixtures, source checks, build validation, and rendered QA form the internal publication gate. Waseem Sial, External Reviewer and Engineer, is listed for ongoing external review; no completed review date is claimed.
