Measure once, order once
Run out of tile halfway through a floor and you'll find the dye lot has changed. Order too much concrete and you pay to dump it. Size an air conditioner wrong and you'll be uncomfortable or broke, or both.
The eight calculators here handle the math for building and trade work. They're for homeowners, contractors and estimators, and they run in your browser.
Materials: concrete, tile, paint and lumber
The concrete yield calculator handles slabs, walls, footings and columns in one project. It adds the volumes, applies a waste factor and returns bags or ready-mix yards, with a cost estimate. Order about 5 to 10 percent extra. Ground is never perfectly level.
The tile calculator adds up several areas, accounts for grout lines and layout waste, and returns tiles, boxes and leftovers. Diagonal layouts waste more than straight ones. Buy an extra box for repairs later, because tile lots vary.
The paint calculator takes wall dimensions, subtracts doors and windows, and multiplies by coats. Texture and ceilings change the numbers. It returns gallons, cans and cost. Rough walls soak up more paint than smooth ones.
The board foot calculator multiplies thickness by width by length, divided by 144, for each lumber row. A board foot is a unit of volume. Add waste for knots and cuts, since you can't use every inch of a board.
Electrical: wire size and voltage drop
Wire that's too thin overheats. The NEC wire size calculator finds the minimum AWG based on ampacity tables in NEC 310.16 and the voltage drop over your run. It also applies conduit derating when you bundle conductors together.
The voltage drop calculator shows volts lost and the percentage, with a full AWG comparison table. The common guideline is 3 percent on a branch circuit and 5 percent total for feeder and branch combined. Long runs to a shed or well pump fail this test more often than people expect.
Codes differ by location, and local amendments apply. Use these results to plan and talk to your electrician. Don't use them to skip a permit or inspection. Electrical mistakes start fires.
HVAC and solar
The HVAC sizing calculator estimates BTU and tonnage from climate, insulation, windows and occupancy, for a single room or the whole home. Bigger isn't better. An oversized unit cycles on and off, which wastes energy and leaves the air damp.
A professional load calculation (called Manual J in the US) beats any quick estimate. Use this tool to sanity check a contractor's quote. If they propose twice the tonnage you calculated, ask why.
The solar panel estimator uses NREL peak sun hours for your area to estimate daily and yearly kWh. It adds payback, a 25-year ROI chart and CO2 offset. Shade, roof angle and panel aging change real output, so build in a margin.
Tips from the job site
Measure twice. Then have someone else measure once. A fresh pair of eyes catches the typo.
Round up on materials with long lead times. Waiting two weeks for one more box costs more than keeping a spare.
Keep a note of every assumption. When the quote changes, you'll know whether the math or the scope shifted.
Store results with the project. A photo of the output next to your sketch beats a memory.
Know the limits
These are estimating tools. Structural, electrical and mechanical work often needs a licensed pro, and many places require permits. When safety is involved, follow local code over any calculator.
Waste, rounding and ordering
Always order more than the exact figure. Tile needs 10 percent extra for cuts and breakage, more for diagonal patterns. Paint depends on the surface; rough walls drink more. Lumber has offcuts you can't use.
Round up to whole units. Nobody sells 3.4 gallons or 7.2 boxes. And keep a spare box from the same batch, since colors vary between production runs and a later match can be hard to find.
Measuring twice
Most mistakes in these tools come from units and not from math. Check that every input uses the same system and whether a size is nominal or actual. A 2 by 4 board isn't two inches by four.
Add waste and a safety margin on top of any result. Ordering a little extra is cheaper than a second delivery.