Guides & how-tos
The practical answers behind the tools — how much material to buy, what size to fit, and the rules of thumb the pros use. Every guide links straight to the calculator that does the math.
How Many Bags of Concrete Do I Need?
Find the volume, then the bags: an 80 lb bag makes about 0.6 cu ft, so it takes roughly 45 to fill a cubic yard. Here is the quick way to size any pour.
Read the guideWhat Size Wire Do I Need? (AWG by Amps)
Two things set wire size: how much current it must carry (ampacity) and how far it runs (voltage drop). Here is the chart, and when to go bigger.
Read the guideHow Many BTU Do I Need to Cool a Room?
Start at about 20 BTU per square foot, then adjust for sun, ceiling height, people and kitchens. Here is the chart and why bigger is not better.
Read the guideWhat Slope Does a Drain Pipe Need?
The classic rule is ¼ inch of fall per foot of run — but big pipes want less, not more. Here is why, with a slope-by-size chart.
Read the guideHow Many Solar Panels Do I Need?
Divide your daily energy use by your sun hours and losses to get the array in kW, then by the panel wattage for the count. Here is the method, with a chart.
Read the guideWhat Size Breaker Do I Need?
Find the load in amps, add 25% if it runs for hours, then round up to the next standard breaker. Here is the chart — and the wire-size rule people miss.
Read the guideWhat Size Furnace or Heater Do I Need? (Heating BTU)
Start near 30 BTU per square foot, then adjust for insulation, windows and how cold your winters get. Here is the chart, plus why heat pumps need extra care.
Read the guideWhat Size Water Heater Do I Need?
Plan on about 13 gallons of stored hot water per person, then match the tank to your peak-hour demand. Here is the chart, plus why recovery time matters.
Read the guideHow Many Bricks Do I Need?
Wall area times bricks per square foot, minus your openings, plus waste. Here is the method, the per-square-foot numbers, and the mortar you will need.
Read the guideHow Much Gravel Do I Need?
Area times depth gives the volume, then multiply by density for tonnage. Here is the method, the depths to use, and how many tons a cubic yard weighs.
Read the guideHow Much Paint Do I Need?
Wall area times coats, divided by the coverage on the can. Here is the method, the coverage to expect, and why primer on bare surfaces pays for itself.
Read the guideWhat Size Battery Do I Need for Backup?
Load in kW times backup hours gives the usable energy; divide by depth of discharge and inverter efficiency for the rated kWh. Here is the method, with an example.
Read the guideWhat Size Generator Do I Need?
Add the running watts, add the largest motor’s starting surge, convert to kVA and add headroom. Here is the method — and why surge watts decide the size.
Read the guideWhat Size Duct Do I Need? (CFM to Duct Size)
Airflow divided by target velocity gives the area; the diameter follows. Here is the method, the speeds to design to, and the standard sizes to round up to.
Read the guideWhat Size Pipe Do I Need? (Water Supply)
Flow rate divided by target velocity gives the bore; round up to the next NPS size. Here is the method, the safe water speeds, and typical fixture flows.
Read the guideHow Much Topsoil Do I Need?
Area times spread depth gives the volume; convert to bags or tons by material. Here is the method, the depths to spread, and when bulk beats bags.
Read the guideHow Long Does Solar Take to Pay Off?
Payback is the year your running savings equal the up-front cost. Here is what drives it — self-consumption, export rate and rising utility prices — with an example.
Read the guideHow Do I Calculate Voltage Drop? (And Size for a Long Run)
Voltage drop grows with current and distance and shrinks with conductor size. Here is the formula, the limits to design to, and when to go up a gauge.
Read the guideWhat Size Extractor Fan Do I Need? (CFM by Room)
Room volume times air changes per hour, divided by 60, gives the CFM. Here is the method, the air-change rates per room, and how to pick the fan.
Read the guideHow Do I Size a Pump? (Total Dynamic Head)
A pump is chosen by flow and head, not by its max-head sticker. Here is how to add up total dynamic head and pick from the curve, with an example.
Read the guideHow Many Pavers Do I Need?
Area divided by one paver’s face, plus waste. Here is the method, the bedding sand you will need, and why the sub-base and edge restraints matter.
Read the guideHow Much Sod Do I Need?
Measure the area, add 5–10% for cutting, then convert to pieces or pallets. Here is the method, plus how to prepare the bed and finish with dressing.
Read the guideHow Do I Convert kW to Amps?
Amps come from watts, volts and the power factor — with a √3 for three-phase. Here are all three formulas, with a worked example and the power factor to pick.
Read the guideWhat Is Ohm's Law? (Volts, Amps, Ohms, Watts)
Voltage equals current times resistance — and from any two of volts, amps, ohms and watts you can find the rest. Here is the law, the power formulas, and the memory triangle.
Read the guidekVA vs kW: What's the Difference?
kW is the work done; kVA is what the supply must deliver to do it — and the power factor is the gap. Here is how they relate, and how to convert between kVA, kW and amps.
Read the guideHow Much Does an Appliance Cost to Run?
Watts, times hours, times your rate per kWh — that is the whole formula. Here is how to find an appliance’s wattage, and which ones quietly dominate the bill.
Read the guideHow Many Tiles Do I Need?
Area divided by one tile’s area, plus a cutting allowance — then the adhesive and grout follow from the same measurement. Here is the method, with a worked example.
Read the guideHow Much Plaster Do I Need? (Cement & Sand)
Wall area times coat thickness gives the volume; the mix ratio splits it into cement and sand. Here is the method, the mix to use, and why thickness matters most.
Read the guideHow Do I Calculate Stairs? (Rise and Run)
Divide the total rise by a comfortable riser, round to whole risers, then divide back for the exact one. Here is the method, the comfort rule, and the code limits.
Read the guideHow Do I Calculate Roof Pitch? (And Rafter Length)
Pitch is measured three interchangeable ways — rise-in-12, degrees and percent — and the same slope gives rafter lengths. Here is how to convert and measure it.
Read the guideHow Many Fence Posts, Rails and Pickets Do I Need?
A fence is a few counts off the run length: posts and concrete, plus the rails and pickets a wood fence needs. Here is the whole material list and how each part is worked out.
Read the guideHow Many Sprinkler Zones Do I Need?
Zones come from flow, not yard size: your supply only runs so many heads at once. Here is how to measure the supply and work out how many zones you need.
Read the guideHow Many Plants Do I Need? (Plant Spacing)
Plants per square foot is 1 ÷ spacing², times the bed area — and staggered rows fit about 15% more. Here is the method, plus the spacing each plant type wants.
Read the guideWhat Angle Should Solar Panels Be? (Tilt & Direction)
Tilt at roughly your latitude and face the equator — and being a bit off costs surprisingly little. Here are the rules, the numbers, and what your roof pitch costs.
Read the guideHow Much Power Does a Solar Panel Produce?
Panel size times your sun hours times the losses gives the daily kWh. Here is the formula, what peak sun hours mean, and the yield to expect for your region.
Read the guideWhat Size Charge Controller Do I Need? (MPPT)
Controller amps are array watts ÷ battery voltage, plus a safety factor. Here is the method, why 48 V needs a smaller controller, and the cold-voltage check.
Read the guideHow Many Solar Panels Can I Wire in Series? (String Sizing)
A string must stay inside the inverter’s voltage window all year — and panel voltage swings with temperature. Here is how cold and heat set the max and min string length.
Read the guideHow Long Does a Battery Take to Charge?
Charge time is the energy to put back divided by the charging power. Here is the method, the efficiency to use per chemistry, and why lead-acid takes longer.
Read the guideHow Much Refrigerant Does a Long Line Set Need?
Every foot of liquid line beyond the factory charge holds refrigerant you must add. Here is the per-foot rate by line size, and how to weigh in the extra.
Read the guideWhat Are Superheat and Subcooling? (Charging an AC)
Superheat and subcooling are the two numbers that reveal a system’s charge — each a line temperature versus its saturation temperature. Here is how to read and use them.
Read the guideHow Much Does Air Conditioning Cost to Run?
An aircon’s draw is its cooling capacity divided by its efficiency; the cost is that times hours times your rate. Here is the method, and how to run it for less.
Read the guideWhat Size Rainwater Tank Do I Need?
How much you can catch is a formula from roof and rainfall; the tank size is about the dry spell you must bridge. Here is both, with the numbers that trip people up.
Read the guideHow Long Does Water Take to Heat? (And What It Costs)
Heating water takes a fixed energy per gallon per degree, so the time is that energy over the element power. Here is the method, the cost, and how to save on it.
Read the guideHow Do I Calculate Square Footage?
Square footage is length × width for a rectangle — and every other shape has its own one-line formula. Here they all are, plus how to measure an awkward room.
Read the guideHow Many Rolls of Wallpaper Do I Need?
Count the vertical drops around the room, see how many come off one roll, and divide — then let the pattern repeat and a waste allowance set the final number. Here is the method.
Read the guideBTU to kW and Tons: How to Convert (with Chart)
Two divisions do it: BTU/hr ÷ 3,412 gives kilowatts, and BTU/hr ÷ 12,000 gives tons. Here are the formulas, a chart of the common sizes, and the one unit gotcha.
Read the guideHow Long Will My Battery Last? (Backup Runtime)
Runtime is usable energy divided by the load — and usable is less than the label, once you allow for depth of discharge and inverter losses. Here is the method.
Read the guideWhat Size Cable Do I Need for Solar? (DC Wire)
Solar DC wire is sized on voltage drop, and at 12/24/48 V that is unforgiving. Here is the formula, the drop to allow, and why low voltage needs fat copper.
Read the guideHow Many Lumens Does a Room Need? (And How Many Lights)
Multiply the floor area by the light level the room needs, allow for losses, then divide by one fitting’s output. Here is the method, with the levels to design to.
Read the guideHow Much Water Does a Pipe Hold?
A pipe holds a cylinder of water: bore area times length. Here is the method, the per-foot figures, and why it decides your wait for hot water at the faucet.
Read the guideWater Pressure Units: psi, bar, kPa and Feet of Head
psi, bar, kPa and feet of head all describe the same pressure — converting is one multiplication. Here are the factors and the benchmarks to judge a reading.
Read the guideHow Much Rebar Do I Need? (For a Slab)
Rebar is a grid: bars each way at a set spacing. Count them, total the length, and the bar size gives the weight. Here is the method, with the laps people forget.
Read the guideHow Much Asphalt Do I Need? (Driveway Tonnage)
Asphalt is bought by weight: volume times density gives the tons. Here is the method, the tons-per-yard rule, and how thick to lay it over the base.
Read the guideHow Many Deck Boards Do I Need?
Deck boards are area divided by the area one board covers — its width plus the gap. Here is the method, the gap to leave, and the joists and screws that follow.
Read the guideHow Many Sheets of Drywall Do I Need?
Drywall is area divided by sheet size, plus waste — and the screws, mud and tape follow from the same area. Here is the method and the finishing materials.
Read the guideHow Do I Calculate Board Feet?
A board foot is a volume, not a length: thickness × width × length ÷ 12. Here is the formula, the nominal-vs-actual trap, and the linear-to-board-feet conversion.
Read the guideHow Much Sand Do I Need?
Sand is area times depth times density, plus a compaction allowance — and the type changes the answer. Here is the method and which sand to pick.
Read the guideHow Much Limestone Do I Need? (#57, Crusher Run)
Crushed limestone is area × depth × density — but the grade decides the job. Here is the method, and which of #57, crusher run and screenings goes where.
Read the guideHow Much Rip Rap Do I Need? (Erosion Control)
Rip rap armours the sloped face, which is longer than the ground it covers. Get that slope factor right, pick a layer, and the tonnage follows. Here is the method.
Read the guideHow Many Retaining Wall Blocks Do I Need?
A block wall is courses high times blocks per row. Add the cap row and the gravel pad and you have the order. Here is the method, plus the base and drainage.
Read the guideHow Much Does Carpet Installation Cost?
Installed carpet is three costs — carpet, padding and labor — and the carpet you buy is more than the floor, because of roll-width waste. Here is how it all adds up.
Read the guideHow Much Flooring Do I Need? (Laminate, Hardwood, Vinyl)
Flooring is sold by the box: room area divided by one box’s coverage, plus waste, rounded up. Here is the method for laminate, hardwood and vinyl plank, and the numbers to use.
Read the guideHow Many Wires Can I Put in a Conduit? (Conduit Fill)
The NEC caps how full a conduit can be: 53% for one wire, 31% for two, 40% for three or more. Here is how to work it out from the conductor and conduit areas.
Read the guideHow Many Bundles of Shingles Do I Need? (Roof Squares)
Roofing is measured in squares (100 sq ft). Take the footprint, multiply by the pitch factor for the real roof area, then squares and bundles. Here is the method.
Read the guideHow Many Metal Roofing Panels Do I Need? (& Cost)
Metal roofing is counted by the panel: roof area divided by one panel’s coverage. Here is the method, the coverage widths, the screws, and the cost per square.
Read the guideHow Much Gutter Do I Need? (& How Many Downspouts)
Gutter runs along the eaves, and you want a downspout every 30–40 ft. Here is how to work out the run, the downspouts and the sections, and what it costs.
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