Concrete countertop calculator

Two ways to build the same top: a solid cast poured to full thickness, or a 3/4 in GFRC shell that uses half the concrete and lands under half the weight. This runs both, with the finished weight your cabinets have to carry.

01 How the math works

A countertop is a slab, so the raw math is the plain one: length × width × thickness. What changes between the two build methods is the number that goes in the thickness slot.

A solid, wet-cast top is poured to its full thickness — 1.5 in is the residential standard — and reinforced with a layer of steel at mid-depth. GFRC (glass-fiber-reinforced concrete) is built as a 3/4 in fiber-loaded backer coat behind a thin face coat, with the visible edge formed by a return rather than solid concrete. The same-looking top uses half the concrete, and alkali-resistant (AR) glass fiber does the job the steel would.

The bag count uses the countertop-mix yield of 0.60 ft³ per 80 lb bag (Quikrete Countertop Mix No. 1106). GFRC is usually a from-scratch or specialty blend with its own yield, so read its bag figure as a volume match, not a shopping list — the cubic-foot number above it is the part that carries over.

02 Worked example

Take a kitchen island top: 6 ft × 3 ft × 1.5 in (1.83 m × 0.91 m × 38 mm) — 18 ft² (1.67 m²) of surface.

Solid cast: 18 ft² × 1.5 in = 2.25 ft³ (0.0637 m³). With 10% waste, that's 5 bags of 80 lb countertop mix. The finished top weighs about 337 lb (153 kg) — 18.75 lb/ft² — carrying one layer of steel mesh across roughly 18 ft².

GFRC: the same top as a 3/4 in shell is 18 ft² × 0.75 in = 1.125 ft³ (0.0319 m³), or 3 bags of mix. It weighs about 144 lb (65 kg) — 8 lb/ft² — with roughly 4.3 lb (2.0 kg) of AR glass fiber at 3% of mix weight and no steel. Same top on the counter, a little under half the weight to get up onto the cabinets.

03 When this calculator is wrong

Both figures are the flat surface only. Three things move the real number, and the weight line is doing more work than it looks.

• The GFRC volume is the shell, not the edge. A GFRC top can look two inches thick and weigh like the three-quarter-inch shell it actually is — that's the point of building it that way — but the edge return and any stiffening ribs are extra concrete the shell figure skips. On the 6×3 example, a 3/4 in edge return around the 18 lin ft perimeter adds about 6% to the volume; ribs across a long unsupported span add more.
• The weight is a total, not a support check. A solid 1.5 in top puts 18.75 lb/ft² on the cabinet run — the 18 ft² island is 337 lb concentrated on the bases and on any seating overhang. Standard cabinets carry a top-mounted slab fine; an unsupported overhang for stools does not, and needs steel or corbels under it. This is where the GFRC weight of 8 lb/ft² earns its keep — it's the difference between a top the cabinets carry and one that needs added structure.
• Sink and cooktop cutouts don't come off the order. A typical undermount sink knockout is about 24 × 18 in (3 ft²), but you still cast that area and cut it out, and the piece isn't reusable. Order for the full rectangle. The waste factor is what covers the trimmed material, not a reason to subtract it.

04 What to do with the result

Order the rounded-up number, not the raw one. The cost of returning an unused bag is small; the cost of running the batch short partway through a cast — a cold joint down the middle of a finished top — is the kind you pay once. When the order is close to a bag boundary, round up one more.

The exception is colour. If you're integrally pigmenting the mix, batch every bag for one top together or buy the pigment for the whole job at once — a second batch mixed later won't match, and a countertop is exactly the visible, single-piece surface where a mismatched dye lot shows. That's the one place buying to the number, once, beats rounding up.

For a solid cast, the bag count above is the buy. For GFRC, match the cubic-foot volume with your chosen blend and add AR glass fiber at about 3% of mix weight. Either way, countertop mix reaches 4,000 psi at 7 days and 6,000 psi at 28 — fine to strip and handle inside a day or two, but treat it gently until it has cured.

05 Common questions

How thick should a concrete countertop be?

1.5 in (38 mm) is the residential standard for a solid cast, with 2 in (51 mm) used for a heavier look. GFRC reaches the same finished thickness with a 3/4 in (19 mm) shell and a faked edge, which is how it saves the weight.

How much does a concrete countertop weigh?

A solid 1.5 in top runs about 18.75 lb/ft² (91.6 kg/m²) — the 18 ft² island example is roughly 337 lb (153 kg). GFRC at a 3/4 in shell is about 8 lb/ft² (39 kg/m²), so the same island lands near 144 lb (65 kg).

How many bags of concrete for a countertop?

At 0.60 ft³ per 80 lb bag, an 18 ft² top at 1.5 in solid is 5 bags with 10% waste. The GFRC shell of the same top is 3 bags of volume — though GFRC is usually a specialty blend, so match the cubic feet rather than the bag brand.

Do concrete countertops need rebar or wire mesh?

A solid cast does: a layer of steel mesh or small bar at mid-depth, galvanized or coated so it can't rust and stain through the surface. GFRC does not — the AR glass fiber blended in at about 3% of mix weight is the reinforcement, which is a large part of why it can be built so thin.

Is GFRC better than a solid pour?

Not better, lighter. GFRC uses about half the concrete and weighs under half as much (8 vs 18.75 lb/ft²), which matters for big spans, cantilevers, and getting a finished top up a staircase. The trade is a more involved layered build. A small, well-supported top is fine as a solid cast.