Calculators
Subfloor and prep calculators
Plywood and OSB, joists, screed, leveling compound, moisture barriers, insulation, radiant heat and height transitions.
A finished floor is only as good as what is under it. Tile cracks over a subfloor that flexes, plank floors telegraph every hump in the slab, and carpet on a damp concrete floor smells within a year. The quantities in this group are the ones that fix those problems before the flooring arrives: sheets of plywood or OSB, bags of leveling compound by the depth that actually needs filling, rolls of vapor barrier with their overlaps, batts of insulation between the joists, and the electric mat that turns a cold bathroom floor warm.
The arithmetic is mostly area by coverage, but the coverage depends on a depth or a spacing that people guess at. A quarter inch of leveler over a whole room is a very different order from feathering a few low spots, and 16 inch joist spacing takes a third more joists than 24 inch. Each calculator asks for that one figure and shows what it does to the count. Where the question becomes structural, as it does for joist spans, the tool counts pieces and points you to the span tables and your local code; it does not design the floor.
8 calculators
Subfloor Plywood / OSB Calculator
Sheets of plywood or OSB subfloor or underlayment, the screws on the APA schedule and the adhesive.
Floor Joist Count Calculator
Joists, rim boards, hangers and blocking for a floor at a given spacing; sizing is left to the span tables.
Floor Screed Calculator
Volume and bags of sand-cement screed from the area and depth, with the weight and the ready-mix figure.
Self-Leveling Compound Calculator
Bags of self-leveling underlayment from the area and the average depth, with the primer.
Moisture & Vapor Barrier Calculator
Rolls of polyethylene with the overlaps and tape, or gallons of liquid barrier, for a floor on a slab.
Underfloor Insulation Calculator
Bags of batts between the joists or sheets of foam board, by R-value, with the wire supports.
Radiant Floor Heating Calculator
The heated area after fixtures and wall gaps, the mat size, the watts and amps, and the circuit it needs.
Floor Height Difference Calculator
The height difference between two floors, the transition strip it calls for, and the build-up to level it.
Guides for this topic
Removing an old floor: cost, time, debris and what to watch for
Taking up the old floor is the dirty, heavy part of every flooring job. Carpet comes up in an afternoon; tile set in mortar takes days and weighs a ton. Here is what to expect for each.
Large-format tile: what it needs before you order
Big tiles look clean and save grout, but they need a flatter floor, deeper mortar, leveling clips and careful layout. Here is what changes in the order and on the floor when a tile side reaches 15 inches.
Electric radiant floor heating: area, watts, circuit and running cost
Electric floor heat is sized by the open floor you stand on, not the room. Here is how to work out the heated area, the load on the circuit and the monthly bill for a bathroom and a kitchen.
How much self-leveling compound? Depth, bags and primer
A bag of leveler covers about 47 sq ft at 1/8 inch and half that at 1/4 inch. The depth is everything, and it is the number most people guess. Here is how to measure it and turn it into bags.
Subfloor preparation: flat, dry, sound and clean before any floor
Most flooring failures start underneath. A floor that is flat, dry, stiff and clean takes any finish; one that is not ruins the best plank or tile. Here are the checks and the fixes.
Underlayment: which type, how many rolls, and when you need none
Underlayment smooths small flaws, softens footfall, cuts noise and keeps slab moisture out. The right one depends on the floor above and the subfloor below, and sometimes the right one is none.
Questions people ask
- How many sheets of plywood for a 400 square foot subfloor?
- A 4 × 8 ft sheet covers 32 sq ft, so 400 sq ft is 12.5 sheets before waste; with the usual 10% for cuts and staggered joints you buy 14. On 16 inch joists each sheet takes about 43 screws at 6 inch edge and 12 inch field spacing, so 14 sheets need roughly 630 screws, two 5 lb boxes, and about five tubes of construction adhesive at one tube per three sheets.
- How many bags of self-leveling compound for 1/4 inch over 200 square feet?
- A 50 lb bag of a typical self-leveler covers about 47 sq ft at 1/8 inch (Custom LevelQuik RS lists 46 to 48), so about 23.5 sq ft at 1/4 inch. A full 1/4 inch over 200 sq ft is 8.5 bags before waste and 10 with a 10% mixing allowance, plus primer; if only a third of the room is low, 4 bags. Measure the actual depth with a straightedge before you order; the compound is expensive and heavy.
- How many joists for a 12 foot span at 16 inches on centre?
- Joists are counted along the length of the room, not the span: a room 16 ft long at 16 inch spacing needs 16 × 12 ÷ 16 + 1 = 13 joists, plus the two rim boards at the ends. Whether a 2 × 8 or a 2 × 10 can carry a 12 ft span is a question for the IRC span tables and your local code, not for a quantity calculator.
- What size radiant heating mat for a bathroom?
- Mats cover the open floor only, not under the vanity, toilet or tub, and they stop a few inches from every wall. A 5 × 8 ft bathroom with about 12 sq ft taken by the vanity, toilet and tub edge, and a 4 inch gap kept from the walls, has about 20 sq ft of heatable floor, which at 12 W per sq ft is a 240 W mat drawing 2 A at 120 V. The radiant calculator subtracts the fixtures and sizes the mat and the circuit.
- How do I deal with a height difference between two floors?
- Add up each floor's build-up (underlayment plus flooring thickness) and compare. A difference under about 1/8 inch is hidden by a T-moulding; up to about 1/2 inch takes a reducer strip; more than that is either a ramp strip or extra underlayment on the low side. The floor height calculator does the sum for any two materials.
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