How do I work out the square footage of an L-shaped countertop?

Measure each run along its wall from the corner, multiply by the depth, and take the corner square off once — otherwise it is counted in both runs. Runs of 120" and 96" at the standard 25 1/2" depth are (120 + 96 − 25 1/2) × 25 1/2 = 4857.75 sq in, which is 33.73 sq ft. A U takes the corner off twice.

What is the standard countertop depth?

25 1/2" from the wall to the front edge over standard 24" deep base cabinets — the extra inch and a half is the overhang past the cabinet doors. An island is usually measured overhang included, and a seating overhang of 10" to 15" is common. Use the depth your cabinets and your fabricator actually give you; the calculator takes any figure.

Do you subtract the sink cut-out from the square footage?

Usually not. Most fabricators price the finished top before the holes are cut — the slab is bought and machined either way — and then charge for each cut-out on top. That is why the calculator counts the cut-outs by default and only takes them off when asked, which is right for something bought by the area it covers, such as tile on a plywood top.

How many slabs will my countertop need?

It depends on how the pieces fit, not on the total area. An L of 120" and 96" runs with a 72" × 36" island and a 4" backsplash is 57.7 sq ft — and on 120" × 55" slabs those pieces come out of two. A run longer than the slab has to be seamed. That is a geometric minimum: a fabricator matching veins across a seam may use more, and their layout is the one you pay for.

Countertop calculator: square feet, backsplash and slabs

Square footage for a straight, L or U countertop with the corner counted once, an island and the backsplash, and how many slabs of a given size the pieces come out of.

Layout

Run A

The wall with the corner, end to end

Depth

= 25.5"

Run B

Along its wall, from the corner

Island length

0 for no island

Island width

Overhang included

Advanced settings4" backsplash · cut-outs not deducted · 120" × 55" slab · 1/8" kerf

Backsplash height

4 is the usual stone splash; 0 for none

Sink and cooktop cut-outs

Most fabricators price the top before the holes, so the default leaves them in

Sink cut-out width

From the sink's template; 0 for none

Sink cut-out depth

Cooktop cut-out width

0 for a range or none

Cooktop cut-out depth

Slab length

Ask the yard; quartz is often 120

Slab width

Often 55; jumbo nearer 65

Saw kerf

= 0.125"

Countertop area

51.7sq ft

Slabs

Backsplash

6.0 sq ft

Top and splash

57.7 sq ft

Where the number comes from

Run A21.25 sq ft: 120" × 25 1/2"
Run B12.48 sq ft — the corner already counted in run A: 70 1/2" × 25 1/2"
Island18.00 sq ft: 72" × 36"
Splash A3.33 sq ft: 120" × 4"
Splash B2.67 sq ft: 96" × 4"
Cut-outsNot deducted — the stone is bought and cut either way: 5.04 sq ft
Seams forced by slab lengthEvery piece fits a slab whole: 0

The pieces on 2 slabs

Slab 1

Slab 2

This is the geometric minimum. A fabricator matching veins across a seam, or keeping a run and its splash from one slab, may use more — their layout is the one you are quoted.

Before you get a quote

Measure each run along the wall, from the corner, to where the top will end — past the last cabinet if the top overhangs it. An island's size includes its overhang.

A seating overhang deep enough to sit at may need brackets or corbels underneath. The countertop overhang calculator works out whether it does and how many.

Worked example: an L-shaped kitchen with an island

Runs of 120" and 96" at 25 1/2" deep come to 33.73 sq ft with the corner counted once, and the 72" × 36" island adds 18.00 sq ft, for 51.73 sq ft of top.

A 4" backsplash along 216" of wall is 6.00 sq ft more, 57.73 sq ft in all.

The pieces come out of two 120" × 55" slabs with no seam forced by the slab length.

Countertop: 51.73 sq ft
Backsplash: 6.00 sq ft
Total: 57.73 sq ft
Slabs: 2

These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.

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About countertop area

How do I work out the square footage of an L-shaped countertop?: Measure each run along its wall from the corner, multiply by the depth, and take the corner square off once — otherwise it is counted in both runs. Runs of 120" and 96" at the standard 25 1/2" depth are (120 + 96 − 25 1/2) × 25 1/2 = 4857.75 sq in, which is 33.73 sq ft. A U takes the corner off twice.
What is the standard countertop depth?: 25 1/2" from the wall to the front edge over standard 24" deep base cabinets — the extra inch and a half is the overhang past the cabinet doors. An island is usually measured overhang included, and a seating overhang of 10" to 15" is common. Use the depth your cabinets and your fabricator actually give you; the calculator takes any figure.
Do you subtract the sink cut-out from the square footage?: Usually not. Most fabricators price the finished top before the holes are cut — the slab is bought and machined either way — and then charge for each cut-out on top. That is why the calculator counts the cut-outs by default and only takes them off when asked, which is right for something bought by the area it covers, such as tile on a plywood top.
How many slabs will my countertop need?: It depends on how the pieces fit, not on the total area. An L of 120" and 96" runs with a 72" × 36" island and a 4" backsplash is 57.7 sq ft — and on 120" × 55" slabs those pieces come out of two. A run longer than the slab has to be seamed. That is a geometric minimum: a fabricator matching veins across a seam may use more, and their layout is the one you pay for.