Who built the wall.

What PlankMath is

PlankMath is 18 project calculators that turn a room's measurements into a store-ready buy list: paint, wallpaper, flooring, drywall, decking, fencing, mulch, gravel, concrete, AC sizing, electric heat, and a bench of electrical tools for wire size, voltage drop, conduit fill, service load, generator sizing and lighting layout. You get the actual thing you put in the cart (three gallons and a quart, fourteen boxes, four and a half cubic yards, thirty-one posts and ninety-three rails) and no area in square feet that you then have to convert yourself while standing in an aisle.

Every calculator is free and works without an account. There is an optional Journeyman membership for people who want to save projects across visits and browse without ads, but nothing behind it changes what the calculators compute.

Why it exists

Most project calculators stop one step short of useful. They tell you that your room is 384 square feet and leave you to work out how many gallons that is, at what coverage, with how many coats, rounded to what the store actually sells. People get that last step wrong, and getting it wrong means either a second trip or a shelf of paint you will never open.

The other common failure is the opposite: a calculator that hands you a confident number with no indication of what it assumed. A paint estimate is meaningless without knowing the coverage rate it used, because coverage varies from about 250 to 450 square feet per gallon depending on the paint and the surface. PlankMath prints its assumptions on every result and lets you change them, because your can's label beats any default we could pick.

Where the numbers come from

Every formula on this site comes from a published trade convention or a manufacturer-stated figure. The defaults are the ones a contractor would use as a starting point:

• Paint at 350 sq ft per gallon per coat (the middle of the range printed on most interior latex labels), broken into gallons and quarts because that is how it is sold.
• Flooring with a 10% waste allowance, which is standard for straight lay-ups; diagonal and herringbone patterns want 15% or more.
• Wallpaper at 56 usable square feet per single roll after trimming, with a 15% pattern-repeat allowance.
• Mulch and gravel in cubic yards, converted to 2 cu ft bags or to tons at a stated density of 1.4 tons per cubic yard for common gravel.
• Decking on 5.5-inch boards with a 1/8-inch gap, sold in 12-foot lengths, with joists at 16 inches on centre and two screws per board per joist.
• Fencing with posts at 8-foot spacing, three rail levels per section, and pickets at 5.5 inches wide with a 1.75-inch gap.
• BTU from the Department of Energy's 20 BTU per square foot rule of thumb, adjusted for ceiling height, sun exposure, insulation and climate.
• Concrete at the bag yields printed on the bags themselves (0.6 cubic feet from an 80 pound bag), with 10% waste for subgrade that is never as flat as the drawing.
• Drywall at 32 square feet a sheet with 10% waste, 32 screws a sheet, one 4.5 gallon box of compound per 475 square feet, and 37 feet of tape per 100 square feet of board.
• Electrical from the National Electrical Code's own tables: ampacity from Table 310.16, the small-conductor limits of 240.4(D), standard breaker ratings from 240.6(A), conduit and conductor areas from Chapter 9 Tables 4 and 5, and the optional dwelling load calculation of 220.82. Voltage drop uses the standard K of 12.9 for copper and 21.2 for aluminum.

Each of those numbers is adjustable on its calculator, and each result states which value it used. The reasoning behind them is written up in the guides.

What it will not do

• Replace measuring properly. Every calculator is only as good as the numbers you feed it, and the most expensive mistakes on this site start with a tape measure, not with arithmetic.
• Replace an electrician, an inspector or a permit. The electrical calculators read published NEC tables and are useful for planning and for checking a quote. They do not know your local amendments, the code edition your jurisdiction has adopted, what is already on the circuit, or what is behind your wall. Electrical work is permitted, inspected work.
• Substitute for your product's label. Coverage rates vary by brand, sheen, colour and surface. Where your label disagrees with our default, the label is right.
• Account for structural or code requirements. The fence and deck calculators produce a material list, not an engineered plan. Post depth, footing size, span tables and permits are governed by your local building code.
• Hide the assumptions. Anything the calculator assumed is printed on the result, where you can see it and change it.

Who builds it

PlankMath is designed, written and maintained by a single independent developer working as Studio Ropewalk, LLC, a limited liability company registered in North Carolina, United States. The formulas were implemented from published trade standards and manufacturer coverage data, each one tested against worked examples; the guides were written by hand by the same person.

There is no crew behind the counter and no support queue. Messages sent through the contact page are read by the person who wrote the code. If a number on this site is wrong, that is worth knowing and corrections get made, because a bad estimate costs someone real money.

How it is funded

PlankMath is free and stays free. It is paid for by display advertising in fixed slots around the board, plus the optional Journeyman membership, which saves projects across visits and removes the ads. Your measurements are not part of the deal: calculations run in your browser and are never transmitted unless you are signed in and explicitly save a project. The privacy policy covers exactly what is stored and what advertising partners set.

Where to start

Pick a tool off the wall on the home page, or read the guides first if you want to understand what the calculator is going to assume before it assumes it. The measuring guide is the one that saves the most money.