Thinset / Tile Mortar Calculator
Select your trowel notch size and enter area to see how many 50 lb bags of thinset you need. Includes an optional back-buttering adjustment for large-format tile and natural stone.
Area & Options
How the thinset formula works
Step 1 — coverage per bag by trowel notch
3/16 V: 95 • 1/4×1/4: 90 • 1/4×3/8: 64 • 1/2×1/2: 45 • 3/4 U: 44 sq ft / 50 lb bag Source: Custom Building Products VersaBond chart (conservative end). A deeper notch lays more mortar per square foot, so coverage drops. The 1/2″ square notch covers roughly half the area of the 1/4″ notch.
Step 2 — back-buttering adjustment
effective_coverage = coverage / 1.35 (if back-buttering) Back-buttering applies a second skim coat to the tile back, adding roughly 35% more mortar. Required by ANSI/TCNA for large-format (15+ in) tile and natural stone.
Step 3 — bags
bags = ⌈ area × (1 + waste/100) / effective_coverage − 1e−9 ⌉ Bags are always rounded up to whole bags. The −1e−9 guard prevents a phantom extra bag when the division lands exactly on a whole number.
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Frequently Asked Questions
It depends on your trowel notch size. Using a 1/4×1/4 in square-notch (90 sq ft per 50 lb bag), 100 sq ft with 10% waste needs 110 sq ft effective area / 90 = 1.22 bags → 2 bags. A 1/2×1/2 in notch covers only 45 sq ft per bag, so the same 110 sq ft needs 3 bags.
As a rule: small tile (up to 4 in) uses a 3/16 in V-notch; standard 12×12 to 16×16 uses a 1/4×1/4 in square-notch; 12×24 to 18×18 uses 1/4×3/8 in; 18 in and larger uses a 1/2×1/2 in or 3/4 in U-notch. The 1/2 in notch roughly halves coverage compared to the 1/4 in notch, so it uses twice as much mortar per sq ft.
Back-buttering applies a second thin skim coat of mortar directly to the tile back, giving 95%+ coverage contact (required by ANSI A108 for large-format tile and natural stone). It increases mortar consumption by roughly 35%, which this calculator models by dividing effective coverage by 1.35.
Coverage varies with trowel angle, back-buttering, substrate flatness, and how much mortar is collapsed when you set the tile. This calculator uses the Custom Building Products VersaBond chart (conservative end): 90 sq ft/bag for 1/4×1/4 in, 45 for 1/2×1/2 in. Mapei and Laticrete publish similar ranges. The conservative end is intentional — it is better to have a bag left over than to run short mid-job.
Thinset (cement-based mortar) is used on floors, wet areas (showers, tub surrounds), and any large-format tile. Mastic (pre-mixed organic adhesive) is for dry interior wall tile only and fails when wet. This calculator is for thinset mortar only; never use mastic for floor tile or wet areas.