How this calculator works
Thinset coverage depends on the trowel notch: a bigger notch leaves taller ridges and uses more mortar. The calculator divides your area by the coverage a 50 lb bag gives with that notch, adds a waste allowance and rounds up to whole bags.
Coverage figures are the conservative end of typical manufacturer bag charts: about 90 sq ft per 50 lb bag with a ¼″ × ¼″ square notch, 60 sq ft with ¼″ × ⅜″ and 40 sq ft with ½″ × ½″. Metric bags are 25 kg; a smaller bag tops up a short remainder.
area = length × width
thinset lb = area ÷ coverage per 50 lb bag × 50 × (1 + waste)
coverage per 50 lb bag: ¼ × ¼ = 90 sq ft · ¼ × ⅜ = 60 sq ft · ½ × ½ = 40 sq ftWorked example
A 10′ × 8′ floor is 80 sq ft. With 12″ tile and a ¼″ × ⅜″ square-notch trowel, a 50 lb bag covers about 60 sq ft: 80 ÷ 60 × 50 = 66.7 lb, and with 10% waste 73.3 lb. That is one 50 lb bag plus one 25 lb bag for the rest.
Tips before you buy
- Check the coverage chart on the bag you buy; mortars vary, and the chart is the figure to trust.
- Lift a freshly set tile now and then. The back should be at least 80% covered on dry floors and 95% in wet areas; switch to a bigger notch if it is not.
- Back-butter large tiles with a thin skim coat for full contact, and allow a little extra mortar for it.
- Mix only what you can use before the pot life on the bag runs out, and never re-temper thinset that has started to set.
Questions
- Which trowel size do I need?
- A common rule of thumb: ¼″ × ¼″ square notch for tile up to 8″, ¼″ × ⅜″ for tile up to about 15″ and ½″ × ½″ for larger tile. Uneven floors and tile with deep back patterns may need the next size up.
- How much area does a 50 lb bag of thinset cover?
- Typical bag charts give about 90 to 100 sq ft with a ¼″ × ¼″ notch, 60 to 75 sq ft with ¼″ × ⅜″ and 40 to 50 sq ft with ½″ × ½″. This calculator uses the low end of each range so you do not run short.
- Modified or unmodified thinset?
- Polymer-modified thinset is the usual choice for floors and walls over cement board and concrete. Some membranes and tile types call for unmodified mortar; follow the instructions of the membrane or tile manufacturer.
- Can I use premixed mortar instead?
- Premixed adhesives are meant for small wall tile in dry areas. For floors, showers and large tile, use a cement thinset mixed from powder.