Trade guides & how-tos
Clear, plain-English answers to the questions tradespeople, contractors, inspectors and homeowners actually search — how to size it, calculate it, and what the code asks for. Every guide cites its sources and links to a free calculator.
Trade
- Hot water cylinder size — guideWhat size water heater do I need? Pick people + fuel, get the size.
- What is First Hour Rating (FHR)?What is your water heater's First Hour Rating? Enter the tank size, heat input and temperature rise — get the hot water it delivers in the first hour (usable storage + recovery).
- What size water softener do I need?What size water softener do I need? Enter your household size, water hardness and daily use — get the grain capacity to look for, sized to regenerate about once a week.
- How to calculate a pipe offsetHow long do you cut a pipe offset? Enter the offset and pick the fitting angle — get the travel (the diagonal to cut) and run, with the 45° 1.414 multiplier and rolling offsets built in.
- What size expansion tank do I need?What size expansion tank do you need? Enter the water heater or system volume, the temperatures and the fill and relief pressures — get the minimum thermal expansion tank size in gallons or litres.
- What size sump pump do I need?What size sump pump do you need? Do the one-minute pit test — enter the pit diameter and how far the water rises in a minute — and get the gallons-per-hour (or litres-per-hour) capacity your pump must deliver at your lift.
- How much concrete do I need?How much concrete do I need? Enter length, width and thickness — get cubic yards, metres and bags.
- How much concrete do I need for footings?How much concrete do I need for footings or post holes? Enter the shape, dimensions and number of footings — get cubic yards, cubic feet or cubic metres plus the bag count.
- How to calculate the volume of a cylinderWhat is the volume of a cylinder? Enter the diameter and height — get the volume in cubic feet, cubic yards, gallons and litres, using V = π r² h, for upright or on-its-side tanks and partial fills.
- How many gallons is my pool? Volume for every pool shapeHow many gallons is my pool? Enter the shape and dimensions — get US gallons and litres for rectangle, round, oval and kidney pools.
- How to calculate slopeWhat is the slope? Enter rise and run — get the percent grade, the angle in degrees and the ratio (1 in n), plus the length along the slope. For ramps, driveways, drainage falls and land grading.
- How much gravel do I need?How much gravel do I need? Enter length, width and depth — get tons, cubic yards and tonnes for gravel, road base or sand.
- How much asphalt do I need?How much asphalt do I need? Enter length, width and thickness — get the tons (and tonnes) of hot-mix asphalt to order for a driveway, path or car park.
- How to calculate board feetHow many board feet do you have? Enter thickness, width, length and quantity — get the total board footage of lumber, plus the volume in cubic feet or metres.
- How much drywall do I need?How much drywall do I need? Enter your room size and sheet size — get the number of sheets to buy, plus a screw estimate, for walls and ceilings.
- How many tiles do I need?How many tiles do I need? Enter the area and your tile size — get the number of tiles (and boxes) to buy for a floor or wall, with a waste allowance for cuts.
- How much flooring do I need?How much flooring do I need? Enter the room size and your box coverage — get the area to buy and the number of boxes for laminate, vinyl plank, hardwood or engineered floors, with a waste allowance worked in.
- How many fence panels do I need?How many fence panels do I need? Enter your fence length and panel width — get the panel count and posts. Switch to picket mode for posts, rails and individual pickets.
- How much sod do I need?How much sod do I need? Enter your lawn length and width — get the area, square yards, pieces and how many pallets to order for a new lawn.
- How much mulch do I need?How much mulch do I need? Enter your bed size and depth — get the cubic yards (or cubic metres) and the number of bags to buy for garden beds and borders.
- How many pavers do I need?How many pavers do I need? Enter the area length, width and paver size — get the count instantly, with a waste allowance for cuts and breakage.
- How much topsoil do I need?How much topsoil do I need? Enter the area length, width and fill depth — get cubic yards, cubic metres and bag count instantly.
- How much sand do I need?How much sand do I need? Enter length, width and depth — get cubic yards (or cubic metres), weight in tons, and the number of bags to buy instantly.
- How many sheets of plywood do I need?How many sheets of plywood do I need? Enter the area and pick your sheet size — get the sheet count instantly, with a waste allowance for cuts.
- How many rolls of wallpaper do I need?How many rolls of wallpaper do I need? Enter your room size, wall height and roll size — get the rolls to buy, worked out by counting full-height drops with the pattern repeat allowed for.
- How much paint do I need?How much paint do I need? Enter your room size, coats and coverage — get the gallons or litres to buy, with doors and windows deducted and your tin's spread rate allowed for.
- How many bricks do I need?How many bricks do I need? Enter the wall size and your brick size — get the number of bricks to buy for a single- or double-skin wall, with the mortar joint and a waste allowance worked in.
- How many blocks do I need for a retaining wall?How many blocks do I need for a retaining wall? Enter the wall size and your block size — get the blocks, courses, caps and base gravel to order, with a waste allowance.
- How many deck boards do I need?How many deck boards do I need? Enter the deck size, board and gap — get the boards, rows, lineal length and screws to order, with a waste allowance.
- How to calculate roof pitchEnter rise and run — get the pitch (x:12), the roof angle, grade %, slope factor and rafter length.
- How to calculate roofing materialsHow much roofing do I need? Enter your roof footprint and pitch — get the area, the roofing squares and how many bundles of shingles to order.
- How to calculate stairsEnter your floor-to-floor rise — get the number of risers, riser height, run, stringer length and angle.
- How many BTUs do I need?How many BTUs do I need? Enter your floor area, ceiling height and climate — get the air conditioner or heater size in BTU/hr, kW and tons, using the standard sizing rules of thumb.
- How many solar panels do I need?How many solar panels do I need? Enter your monthly kWh, peak sun hours and panel wattage — get the system size in kW and the number of panels to fit.
- How many studs do I need?How many studs do I need? Enter the wall length, stud spacing, corners and openings — get the stud count plus the plate lumber for the wall.
- How much insulation do I need?How much insulation do I need? Enter the wall or attic size and your package coverage — get the area to buy and the number of rolls, batt packs or bags, with a waste allowance worked in.
- How to calculate R-valueWhat is the R-value of your wall, roof or floor? Add each material layer and its thickness — get the total R-value, RSI and U-value, for insulation, walls and attics.
- How to calculate voltage dropWill my wire drop too much voltage? Enter the current, run length and wire size — get the % drop checked against the 3% / 5% guideline, in AWG or mm², copper or aluminium.
- How to calculate water velocity in a pipeHow fast is the water moving in your pipe? Enter the flow rate and inside diameter — get the velocity in ft/s or m/s, checked against the usual design guideline.
- How to calculate friction loss in a pipeHow much pressure does your pipe run lose to friction? Enter the flow, inside diameter, length and material — get the head loss and pressure drop, using the Hazen-Williams equation.
- How to calculate pipe weightHow much does your pipe weigh? Enter the outside diameter, wall thickness, length and material — get the weight in pounds or kilograms, empty and full of water.
- How much water does a household use?How much water does your household use? Enter your people, fixtures and habits — get your water use per day, month and year, with a breakdown of where it goes.
- How to calculate total dynamic head (TDH)How much head does your pump need? Enter the static lift, the pipe friction loss and any required outlet pressure — get the total dynamic head (TDH) in feet or metres, ready to read off a pump curve.
- How to calculate GPM (water flow rate)Measure your water flow rate — time a container filling, get gallons per minute.
- Plumbing unit conversions: mm to inches, bar to psi, °C to °Fmm↔inch, bar↔psi, °C↔°F, L/min↔GPM, L↔gal, m²↔ft² — instant, both ways.
- What size generator do I need?What size generator do I need? Select the appliances you want to run — get the running load, starting-demand peak and the recommended generator size instantly.
- PEX vs copper pipeCost, lifespan, where each wins, and which to use — a plain-English comparison.
- What is the 27 rule for stairs?The 2R+T=24-27in comfort formula for stair proportions — where it comes from and how it differs from your code's minimums.
- What size room does a 5,000 to 24,000 BTU air conditioner cool?Divide the BTU rating by ~20 for the room size: 5,000 BTU ≈ 250 sq ft, 8,000 ≈ 400, 12,000 (1 ton) ≈ 600. Full chart with kW and m².
- How to estimate a construction jobStep-by-step: material takeoff, burdened labor rate, subcontractor costs, overhead and profit markup. Includes a markup benchmark table.
- How to pass a building inspectionHave the permit and approved plans on site, keep rough-in work uncovered, and schedule only when the work is complete. Covers every inspection stage from footings to final.
How these guides are written
Each guide starts from the questions people genuinely ask about a calculation — pulled from real search data, not invented — and answers them in order of how often they're asked. The structure is deliberate: the direct answer first, then the method step by step, then a worked example with real numbers you can follow on paper, then the follow-up questions. Where a building-code rule applies, the guide names the code and edition; where something is a rule of thumb rather than a legal requirement, it says so plainly.
Every guide pairs with a free calculator that does the same math instantly, so you can read the method once and then use the tool for every job after that. Guides are reviewed when codes change and carry a "last reviewed" date — if you spot something out of date, the contact page is the fastest way to tell us.