Thinset & Mortar Calculator
Setting details
Bigger tile needs a bigger notch.
Required for most tile 15 in and larger.
Thinset needed
2 bags
- Coverage per 50 lb bag
- 75 sq ft
- Area to set
- 120 sq ft
- Back-butter allowance
- None
- Effective area
- 120 sq ft
Buy one extra bag — partial bags absorb moisture and go off quickly once opened.
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Thinset coverage chart by trowel size
| Trowel notch | Use it for | Coverage per bag | Bags per 100 sq ft |
|---|---|---|---|
| 3/16" V-notch | Mosaic and small tile | 110 sq ft | 1 |
| 1/4" U-notch | Tile up to 8" | 85 sq ft | 2 |
| 1/4" x 1/4" square | Tile up to 12" | 95 sq ft | 2 |
| 1/4" x 3/8" square | 12" to 16" tile | 75 sq ft | 2 |
| 3/8" x 3/8" square | 16" to 20" tile | 60 sq ft | 2 |
| 1/2" x 1/2" square | Large format 18"+ | 45 sq ft | 3 |
Add roughly 35% when back-buttering. Coverage assumes a flat substrate — an uneven floor can consume half again as much.
Which thinset for which job
| Application | Mortar type | Why |
|---|---|---|
| Ceramic floor over backer board | Modified | Standard bond, forgiving to work |
| Porcelain, any location | Modified | Porcelain is non-porous and needs the polymer |
| Over uncoupling membrane | Unmodified | Membrane is impervious — modified cannot cure |
| Over waterproofing membrane | Per membrane maker | Instructions override the general rule |
| Large format 18 in and up | Modified, LFT/medium bed | Resists slumping under tile weight |
| Glass tile and mosaic | White modified, glass rated | White prevents shadowing through the tile |
Tools the pros use for this job
Notched Trowel
Match notch depth to tile size — this is the #1 coverage variable.
Mixing Paddle
IMAYCC Cement Mixer, 2100W Electric Concrete Mixer with 6 Speed, 110V Handheld Mud Mixer for Motar,Paint, Grout, Plaster, Thinset,Fodder Stirring Tool
$48.99
Check price on AmazonLump-free mortar in two minutes; hand mixing never gets there.
Modified Thinset
Polymer-modified mortar for porcelain and large-format tile.
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Prices as of Oct 10, 2026, 6:33 AM UTC. Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.
Choosing a mortar? See our thinset buying guide — bags worked out with this calculator.
Don't level a floor with thinset — size the pour in the self-leveling compound calculator.
Which notch to comb with? See our tile trowel buying guide.
Do this math on the job site — in the free app
Smart Tile Calculator for Android: offline, in your pocket, free.
How this calculator works
Coverage figures assume a 50 lb bag, a clean flat substrate, and correct trowel technique — combed in one direction, tile pressed and slid perpendicular to the ridges to collapse them.
- Mosaic and small tile: 3/16" V-notch
- Up to 12": 1/4" × 1/4" square notch
- 12"–16": 1/4" × 3/8"
- Large format 18"+: 1/2" × 1/2" plus back-buttering
Back-buttering large tile adds roughly 30–40% more mortar but is required to hit the 95% coverage that warranties and wet areas demand.
Modified vs unmodified is the decision people get wrong. Polymer-modified thinset is the default for porcelain, large format and most floors. Unmodified is correct over an uncoupling or waterproofing membrane, because the membrane is impervious and a modified mortar sandwiched between two non-porous layers cannot cure properly. The membrane manufacturer's instructions override every general rule here.
Mix to a peanut-butter consistency, let it slake for 5–10 minutes, then re-mix without adding water. Thinset stays workable for about 45 minutes on a warm day, so mix half a bag at a time until you know your pace. Once the mortar is sized, the grout calculator covers the other half of your setting materials, and the tile cost calculator rolls both into a project total. Setting large format? The large-format calculator sizes the tile order to match. Not sure which mortar to buy? Read modified vs unmodified thinset; bedding a floor in board first? The cement backer board calculator counts sheets and screws. Which notch to spread it with? The tile trowel size calculator matches the trowel to your tile size.
Frequently asked questions
How many bags of thinset for 100 square feet?
With a 1/4 x 3/8 inch notched trowel, about 75 sq ft per 50 lb bag — so two bags for 100 sq ft. Large-format tile with a 1/2 inch notch needs three.
How much thinset per square foot?
About 0.5 lb per sq ft with a 1/4 x 1/4 inch notch (a 50 lb bag covers ~95 sq ft), 0.67 lb with 1/4 x 3/8 inch (~75 sq ft), and 1.1 lb with a 1/2 inch notch (~45 sq ft). Back-buttering large tile adds roughly 10–15% on top.
Do I need modified or unmodified thinset?
Use polymer-modified for porcelain and most floors. Use unmodified when setting over an uncoupling membrane or when the membrane manufacturer specifies it — check the membrane instructions first.
Can I use extra thinset to level a floor?
No. Thinset is an adhesive, not a leveler. Fix dips with a self-leveling underlayment or patch compound before tiling, or the tile will crack.
How long does thinset take to cure?
Walk on it after 24 hours and grout after 24–48 hours. Full cure is 28 days, which matters before a shower is used or heavy furniture is moved back in. Cold or humid conditions extend all of these.
What size trowel do I need?
Match the notch to the tile: 3/16 inch V-notch for mosaic, 1/4 x 1/4 inch square for tile up to 12 inches, 1/4 x 3/8 inch for 12–16 inch tile, and 1/2 x 1/2 inch plus back-buttering for anything 18 inches and larger.
Related calculators
- Tile Quantity CalculatorHow many tiles you need for any room, wall, or floor — with waste.
- Grout CalculatorPounds of grout by tile size, joint width, and thickness.
- Shower Tile CalculatorWalls, floor, curb, and niche for a full shower surround.
- DITRA CalculatorDITRA rolls, thin-set bags under and over the membrane, and the trowel for each.
- Tile Trowel Size CalculatorWhich notch for your tile size, mortar coverage to check for, and bags of thinset.
- Tile Cost EstimatorTotal project cost: tile, grout, mortar, trim, and labor.
Guides worth reading next
- Modified vs Unmodified Thinset: The 30-Second Answer (and the Long One)Use modified (ANSI A118.4) thinset for porcelain on most substrates; unmodified (A118.1) under Kerdi and Ditra. Neither is waterproof — here's why.
- How to Measure a Room for Tile (Step by Step)Measure any room for floor or wall tile: handle closets and alcoves, convert inches to decimals, subtract fixtures, and add the right waste percentage.
- Choosing the Right Tile Size for Your RoomHow tile size changes a room: when 12×24 beats 4×4, why small rooms can take large tile, grout line math, and the flatness rule for big formats.