Inside the Home Calculators

What the inside the home shelf answers

Once the structure of a room is settled, the questions turn to finishing it: how much floor is there, how many tiles does it take, how much paint, is the staircase actually comfortable to use, and what size heater or air conditioner the room needs. This shelf covers that stage, paint, wallpaper, flooring and heating, with wastage counted honestly rather than left for you to estimate. Four calculators are live, the Square Footage Calculator, the Tile Calculator, the Stair Calculator and the BTU Calculator, and paint and wallpaper calculators are being built next.

Measuring the room first

The Square Footage Calculator is where most jobs on this shelf start. It adds any number of rooms or sections into one project total, in square feet and square metres at once, then applies a flooring or paint waste allowance and, if you give it a price per unit area, a cost estimate. Most awkward rooms are not actually irregular, they are two or three rectangles put together, an alcove, a bay window, a dog-leg landing, and measuring each piece separately and adding them is faster and more accurate than trying to pace out one clever total. For genuinely curved or triangular spaces the Area Calculator on the maths shelf covers the standard shape formulas. Once you have a floor or wall area, everything downstream on this shelf, tiles, paint, wallpaper, inherits that number, so it is worth getting it right before moving on.

Painting and wallpaper

Painting and papering are still being built as dedicated calculators, but the coverage sums are worth knowing now, because they change depending on what's underneath the first coat. Fresh plaster is porous and pulls moisture straight out of full-strength paint, which is why the first coat on new plaster should be a mist coat, standard emulsion thinned with roughly 30% water, rather than the paint applied at full strength. Skip the mist coat and the finish often comes out patchy, because the plaster has drunk the water out of the paint unevenly before it can level. New plaster is generally best left several weeks to dry out fully before a final decorative finish goes on, though a mist coat itself can go on sooner. For wallpaper, coverage depends heavily on the pattern repeat: a large repeat wastes more paper per roll than a plain or small-pattern paper, because each drop has to be cut to match the one before it, so a patterned paper needs a larger safety margin than a plain one when you're working out how many rolls to buy.

Tiling

The Tile Calculator divides the floor or wall area by the effective size of one tile, the tile plus its grout gap, adds a waste allowance, and converts the tile count into boxes using the tiles-per-box figure printed on the product. The waste allowance reflects how tiles are actually cut on site: a straight, grid-aligned lay typically needs about 10% extra, while a diagonal, herringbone or other pattern lay produces more offcuts and usually wants 15% or a little more. Buying the waste allowance up front matters for a reason beyond breakage, tiles from a later production run can differ subtly in shade even within the same product line, and a floor finished with a mixed batch shows the join.

Staircases

The Stair Calculator is a check rather than a shopping list. From a total floor-to-floor rise it works out the number of risers, the riser height, the tread depth (the going), the total run and the stringer length, draws the side profile, and judges the result two ways: against the long-standing 2R + G comfort rule, where twice the riser height plus the going should land somewhere between about 550mm and 700mm, with roughly 630mm as a comfortable middle figure, and against commonly published UK or US building limits, with a plain pass-or-fails verdict. It is useful for sanity-checking a design before you commit timber to it, but building regulations vary by location and situation, so final dimensions belong with your building control body or a professional joiner, not with a calculator.

Heating and cooling

The BTU Calculator sizes an air conditioner or heater from the room's dimensions, insulation quality, sun exposure, occupancy and whether it doubles as a kitchen, and gives the result as an honest range in BTU per hour rather than one falsely precise number, with the equivalent in watts alongside it, one kilowatt is roughly 3,412 BTU/hr. Sizing matters in both directions: an undersized unit runs flat out without ever properly coping, and an oversized air conditioner short-cycles, cooling the room quickly without running long enough to dehumidify it properly, which leaves the air cold but clammy. Once you know the wattage of a unit you're considering, the Electricity Cost Calculator on the everyday shelf will turn that into a running cost per day, month and year.

Getting the quantities right

Measure the room at more than one point before trusting the number, older houses in particular are rarely square, and a wall that looks straight can be out by several centimetres from one end to the other. Keep the units consistent through the whole calculation, metres for the room and feet for the material bought separately in your head is how errors creep in, which is why every calculator on this shelf shows both systems rather than forcing a conversion you have to do yourself. Buy tile, paint and wallpaper with the waste allowance built in and, wherever the finish matters visually, from a single batch or dye lot, because a shortfall discovered halfway through a room is a far more disruptive delay than the cost of the extra material would have been. And treat the Stair Calculator's verdict as a first check, not a sign-off, actual building regulations are a matter for your local authority.

Where to go next

Before any of this, the structural work, walls, roof, sub-base, sits on the Building Materials shelf, starting with the Concrete Calculator. If the project extends outside, a new floor leading onto a patio or deck, the Garden & Landscaping shelf picks up the outdoor half. For a project big enough to need a skip or a full clearance first, see Big Jobs & Projects. The Home & Garden hub lists every shelf if you're not sure where your job belongs.

Frequently asked questions

Why does new plaster need a mist coat before proper paint?

Fresh plaster is porous and draws moisture out of full-strength paint unevenly, which leaves a patchy finish. A mist coat, emulsion thinned with roughly 30% water, seals the surface gently so the following coats go on evenly.

How much extra should I buy for a tiled floor?

About 10% for a straight, grid-aligned lay, and 15% or more for a diagonal, herringbone or other pattern lay, which produces more offcuts. The Tile Calculator defaults to a sensible figure for each and lets you adjust it.

Can the Stair Calculator replace building control sign-off?

No. It checks a design against the 2R + G comfort rule and commonly published UK or US limits so a bad design shows up early, but building regulations vary by area and situation, so the final say belongs with your local building control body or a professional.