DIY and building glossary
Plain-English definitions of the terms that come up in home and DIY projects, with the calculator or guide that uses each one.
A
- Aggregate
Sand, gravel or crushed stone. It is the bulk of concrete and asphalt, and it is used on its own as a compacted base under slabs, pavers and driveways.
See: Gravel Calculator
- Amp (ampere)
The unit of electric current. The amps in a circuit are the watts divided by the volts, so a 1,500 watt heater on 120 volts draws 12.5 amps.
See: Watts to Amps Calculator · How Many Amps Do Common Appliances Use? Amp and Watt Chart
- Ampacity
The most current a wire can carry continuously without overheating. It depends on the size of the wire, the metal (copper or aluminum) and the temperature rating of the insulation and the terminals.
- AWG (American Wire Gauge)
The US system of wire sizes. A smaller number is a thicker wire: 14 AWG is used on 15 amp circuits, 12 AWG on 20 amp circuits and 10 AWG on 30 amp circuits. Every three steps in the gauge doubles the cross-section of the wire, roughly.
See: Wire Size Calculator (AWG for Amps and Distance) · What Size Wire for 15, 20, 30, 40 and 50 Amps? AWG Chart
B
- Base course
The layer of compacted crushed stone under a slab, patio, paver or driveway. It drains water, spreads the load and gives an even surface to build on, and is usually 4 to 6 inches deep for a patio.
See: Paver Calculator · Building a Paver Patio: Base Depth, Sand and Materials Needed
- Board foot
A volume of lumber of 144 cubic inches: a board 1 inch thick, 12 inches wide and 12 inches long. Hardwood and rough lumber are priced by the board foot.
See: Board Foot Calculator · Board Feet Chart: How to Calculate Board Feet of Lumber
- BTU (British thermal unit)
A unit of heat: the heat needed to warm one pound of water by one degree Fahrenheit. Heating and cooling equipment is rated in BTU per hour, and 12,000 BTU per hour is one ton of cooling. One kilowatt-hour is 3,412 BTU.
See: BTU Converter (BTU/h to Watts, Tons, kW and kWh) · BTU Conversion Chart: BTU/h to Watts, kW, Tons and kWh
C
- CFM (cubic feet per minute)
A measure of airflow. Bathroom fans, range hoods and ducts are sized in CFM, and the CFM of a room fan is how much air it moves in a minute.
See: CFM Calculator: Airflow for a Room · Duct Size Calculator: Round and Rectangular Duct from CFM
- Charge controller
A device between solar panels and a battery that stops the battery overcharging. An MPPT controller converts the extra panel voltage into charging current, and is sized by its amp rating and its maximum input voltage.
See: Solar Charge Controller Size Calculator: Amps and Voltage
- Circuit breaker
A switch in the electrical panel that trips when a circuit carries more current than its rating, to protect the wire from overheating. A breaker for a continuous load is sized at 125% of the load.
See: Breaker Size Calculator · How Many Watts Can a 15, 20, 30 or 50 Amp Circuit Handle?
- Conduit fill
The share of the inside of a conduit taken up by the wires. The electrical code allows 40% for three or more wires, 31% for two and 53% for one.
See: Conduit Fill Calculator: How Many Wires Fit in EMT or PVC · How Many Wires Fit in Conduit? EMT and PVC Fill Chart
- Continuous load
An electrical load that runs for three hours or more, such as an electric car charger or a heater. The circuit is sized at 125% of a continuous load, which keeps the load to 80% of the breaker rating.
- Control joint
A groove cut or tooled into a concrete slab so that the concrete cracks along the groove instead of at random. It is cut to about a quarter of the slab thickness and spaced two to three times the thickness in feet.
See: How Thick Should a Concrete Slab Be? Thickness by Use and Cost
- Cord (of firewood)
A stack of firewood 4 feet high, 4 feet wide and 8 feet long, which is 128 cubic feet. A face cord is 4 by 8 feet but only one log length deep, so it is a fraction of a cord.
See: Firewood Cord Calculator · How Many Cords of Firewood Do I Need for Winter?
- Course
One horizontal layer of bricks, blocks, siding or shingles in a wall or roof. A wall is described by its number of courses and the height of each.
See: Brick Calculator
- Cubic yard
A volume of 3 feet by 3 feet by 3 feet, which is 27 cubic feet. Concrete, mulch, soil and gravel are sold by the cubic yard, and bags are sold by the cubic foot.
See: Cubic Yard Calculator · How to Calculate Cubic Yards: Formula, Chart and Examples
- Curing
Keeping concrete damp for the first days after it is poured so that it gains strength and does not crack. Seven days is the usual minimum, and concrete reaches most of its strength in 28 days.
See: Concrete Calculator
D
- Dew point
The temperature at which the air is full of moisture and water starts to condense. A cold window or pipe below the dew point of the room air gets wet.
See: Dew Point and Relative Humidity Calculator · Dew Point Chart: Temperature, Humidity and When Windows Fog
- Duct friction rate
The pressure lost for every 100 feet of duct, in inches of water. Residential ducts are designed at about 0.08 to 0.10, and a higher rate means a smaller, noisier duct.
See: Duct Size Calculator: Round and Rectangular Duct from CFM · Duct Size Chart by CFM: Round and Rectangular Duct Sizes
E
- EMT (electrical metallic tubing)
Thin-wall steel conduit used to protect wires in garages, basements and commercial buildings. It is bent with a hand bender and joined with set-screw or compression fittings.
See: Conduit Fill Calculator: How Many Wires Fit in EMT or PVC
F
- First hour rating
The amount of hot water, in gallons, that a storage water heater can deliver in one hour starting full. It is on the EnergyGuide label, and should match the hot water used in the busiest hour of the home.
See: Water Heater Size Calculator: Tank Size and First Hour Rating · What Size Water Heater Do I Need? Tank Size and First Hour Rating
- Footing
The wide concrete base under a post, a wall or a column that spreads the load over the soil. It goes below the frost line so that frozen ground cannot heave it.
- Frost line
The depth to which the ground freezes in winter. It varies from a few inches in the south to several feet in the north, and footings go below it. Your building department has the local figure.
G
- Geotextile
A woven or felt-like fabric laid under gravel, pavers or a retaining wall. It stops the stone sinking into the soil below and lets water through, and it is sold by the roll (often called landscape fabric or filter fabric).
- GFCI (ground-fault circuit interrupter)
An outlet or breaker that cuts the power within a fraction of a second when it detects a small leak of current, about 5 milliamps, which might be flowing through a person. It is required in wet places such as bathrooms, kitchens and outdoors.
- GPM (gallons per minute)
A measure of the flow of water. Faucets, showers, pumps and tankless water heaters are rated in gallons per minute.
- Grout
The cement or epoxy fill between tiles. Sanded grout is used for joints wider than about an eighth of an inch, and unsanded grout for narrower joints.
See: Tile Calculator · How Much Tile, Grout and Thinset Do I Need?
H
- Hardscape
The hard parts of a landscape: patios, paths, walls, steps and driveways, as opposed to the plants. They are built on a compacted base.
See: Paver Calculator
- Horsepower (HP)
A unit of power equal to 746 watts. Motors are rated in horsepower, and the current they draw depends on the voltage, the efficiency and the power factor.
I
- Inverter
A device that changes direct current from a battery or solar panels into alternating current for household equipment. It is sized by its continuous watts and its surge rating.
See: Inverter Size Calculator: Watts, Surge, Fuse and Battery · What Size Inverter Do I Need? Watts, Surge and Battery Chart
J
- Joist
A horizontal framing member that supports a floor, a ceiling or a deck. Joists are usually spaced 16 inches on center, and their size and span are set by the building code.
See: Deck Board Calculator · Floor Joist Calculator: Joists, Rim Boards and Subfloor Sheets · How Many Floor Joists Do I Need? Spacing, Rim Joists, Sheets
- Joist tape
Self-adhesive flashing tape stuck to the top of deck joists before the boards go on. It keeps water out of the wood where screws and nails go in, which makes the frame last years longer.
K
- Kilowatt-hour (kWh)
The unit of energy that you pay for on an electricity bill: one kilowatt used for one hour. A 1,500 watt heater running for an hour uses 1.5 kWh.
See: Appliance Electricity Cost Calculator · How Much Does It Cost to Run Common Appliances? Monthly Cost Chart
L
- Linear foot
One foot of length, whatever the width or thickness. Trim, fencing, lumber and gutters are often priced by the linear foot.
See: Linear Feet to Square Feet Calculator · Linear Feet to Square Feet: How to Convert (With a Width Chart)
- Lumen
A measure of the amount of light a bulb gives out. An 800 lumen bulb gives about the light of an old 60 watt incandescent bulb.
See: Lumens Calculator
M
- Mulch
A layer of bark, wood chips or other material spread on a garden bed to keep weeds down and the soil moist. Two to three inches deep is usual.
See: Mulch Calculator · How Much Mulch Do I Need? Coverage Chart & Bulk vs Bags
N
- NEC (National Electrical Code)
The standard for electrical wiring in the United States, published every three years by the National Fire Protection Association. States and towns adopt it, often with changes, and an inspector enforces the local version.
- Nominal and actual size
The nominal size of lumber is its name, and the actual size is what it measures after drying and planing. A 2×4 is really 1½ by 3½ inches.
See: Actual Lumber Sizes: Nominal vs Actual 2x4, 2x6, 1x6 and More
O
- Ohm's law
The relation between voltage, current and resistance: volts equal amps times ohms. It lets you find any one of the three if you know the other two.
See: Ohm's Law Calculator (Volts, Amps, Ohms, Watts) · Ohm's Law Formulas: Volts, Amps, Ohms and Watts With Examples
- On center (OC)
The spacing of framing members measured from the center of one to the center of the next. Wall studs are 16 or 24 inches on center.
See: Wall Stud Calculator
P
- Pitch (roof)
The steepness of a roof, given as the rise in inches for every 12 inches of horizontal run. A 6/12 pitch rises 6 inches in 12, which is an angle of about 26.6 degrees.
- Plate compactor
A petrol or electric machine with a vibrating steel plate that packs gravel or sand down hard and flat. A compacted base under a patio, paver path or shed stops the surface sinking later. It can be rented by the day.
- Polymeric sand
Fine sand mixed with a binder that hardens when it is wetted. It is swept into the joints of pavers to lock them together and keep weeds and ants out.
See: Polymeric Sand Calculator · How Much Polymeric Sand Do I Need? Coverage by Paver Size
- PSI (concrete strength)
The compressive strength of concrete, in pounds per square inch after 28 days. Residential slabs and footings usually use 3,000 to 4,000 psi concrete.
See: Concrete Calculator
R
- R-value
A measure of how well insulation resists the flow of heat: the higher the number, the better. Recommended attic R-values run from about R-30 in warm climates to R-60 in cold ones.
See: Attic Insulation Calculator · How Much Attic Insulation Do I Need? R-Value and Depth Chart
- Rafter
A sloping framing member that runs from the ridge of a roof down to the wall. Its length depends on the span and the pitch.
See: Rafter Length Calculator · Rafter Length Chart: Common Rafters by Span and Pitch
- Rebar (reinforcing bar)
Steel bars set in concrete to hold it together when it cracks and to carry tension. The bar number is the diameter in eighths of an inch, so #4 rebar is half an inch.
See: Rebar Calculator
- Rise and run
The vertical and horizontal distances of a slope, a stair or a roof. The slope is the rise divided by the run, as a ratio or a percentage.
- Roofing square
An area of roof of 100 square feet. Roofing is priced by the square, and asphalt shingles are sold in bundles, three to a square.
See: Roofing Calculator · How to Calculate Roofing Squares and Shingle Bundles
S
- SEER (seasonal energy efficiency ratio)
A rating of the efficiency of an air conditioner or heat pump in cooling mode over a season: the cooling output in BTU for each watt-hour of electricity. A higher number is more efficient.
- Sonotube
A trade name for a cardboard tube used as a form for a round concrete pier or footing. It is peeled off after the concrete sets.
- Subpanel
A second electrical panel fed from the main panel, used to supply a garage, a workshop or an addition. It has a feeder of two hot wires, a neutral and a ground.
See: Breaker Size Calculator · What Size Wire for a 60, 100 or 150 Amp Subpanel? Copper and Aluminum
T
- THHN
The most common insulated building wire. It has a thermoplastic, high-heat, nylon-coated insulation, is used in conduit and is rated for 90°C in dry places.
See: Wire Size Calculator (AWG for Amps and Distance) · Conduit Fill Calculator: How Many Wires Fit in EMT or PVC
- Thinset
A cement-based adhesive used to stick tile to a floor or wall. It is spread with a notched trowel, and the size of the notch depends on the tile.
See: Tile Calculator · How Much Tile, Grout and Thinset Do I Need?
- Ton (air conditioning)
A unit of cooling capacity equal to 12,000 BTU per hour. A 3 ton air conditioner removes 36,000 BTU of heat an hour.
See: AC Size Calculator (BTU) · What Size Air Conditioner Do I Need? BTU Chart by Room Size
- Ton and tonne
A (short) ton is 2,000 pounds. A metric tonne is 1,000 kilograms, which is about 2,205 pounds. Gravel and asphalt are sold by the ton in the US and by the tonne in the UK.
See: Cubic Yards to Tons Calculator · Cubic Yards to Tons Conversion Chart for Gravel, Sand and Soil
U
- Underlayment
A layer under a finished material: felt or synthetic sheet under roof shingles, a pad under carpet, or a thin board under a floor. It protects the surface above and helps it lie flat.
See: Roofing Calculator
V
- Volt
The unit of electrical pressure. Household outlets in the US are 120 volts, and large appliances such as dryers and ranges use 240 volts.
- Voltage drop
The loss of voltage along a wire because of its resistance. A long or thin wire drops more, and the usual aim is 3% or less on a branch circuit.
See: Voltage Drop Calculator · Voltage Drop Chart: How Far Can You Run Wire by Size and Amps?
W
- Waste factor
The extra percentage of a material that you buy to cover cuts, breakage and mistakes. It is 5 to 10% for many jobs and more for diagonal tile or patterned materials.
See: Waste Factors Cheat Sheet: How Much Extra Material to Buy
- Watt
The unit of power: the rate at which energy is used. Watts equal volts times amps, so a 120 volt appliance that draws 10 amps uses 1,200 watts.
See: Watts to Amps Calculator · Watts to Amps Conversion Chart for 12V, 120V, 208V and 240V
- Watt-hour and amp-hour
Units for the energy stored in a battery. Watt-hours are the volts times the amp-hours, so a 12 volt, 100 amp-hour battery holds 1,200 watt-hours.
See: Battery Capacity Calculator (Ah to Wh) and Runtime · How Long Will a 12V Battery Last? Ah to Wh and Runtime Chart
This glossary uses US units and terms. Building in the UK? See the UK glossary