Tools
Calculators and converters for everyday work: units, areas and volumes, building, water and food, health, energy and chemistry. They run in your browser, offline too, and each one explains its formula so you can work it out by hand.
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Unit converters
Length, area, volume, mass, temperature, pressure, energy, power, speed, flow and angles, including US customary, imperial and old Castilian units.
- Length Metres, kilometres, inches, feet, yards, statute and nautical miles, and old Castilian measures.
- Area Square metres, ares, hectares, acres, square feet and yards, and the Castilian fanega.
- Volume and capacity Litres, cubic metres, US and imperial gallons, cubic feet, barrels and kitchen measures.
- Mass and weight Grams, kilograms, tonnes, ounces, pounds, stones, apothecary grains and Castilian arrobas and pounds.
- Temperature Degrees Celsius, Fahrenheit, kelvin and Rankine.
- Pressure Pascals, bar, atmospheres, psi, kgf/cm², millimetres of mercury and metres of water column.
- Energy Joules, calories and kilocalories, watt hours and kilowatt hours, BTU, kilogram-force metres and tonnes of oil equivalent.
- Power Watts, kilowatts, metric horsepower, mechanical horsepower, kcal/h, BTU/h and tons of refrigeration.
- Speed Metres per second, kilometres per hour, miles per hour, knots and feet per second.
- Flow rate Litres per second, minute or hour, cubic metres per hour or per day, and gallons per minute.
- Angles Degrees, arcminutes and arcseconds, radians, gons, turns and hour angle.
Geometry and measurement
Areas, volumes, triangles, slopes and indirect measurements to survey land, tanks and building work without modern instruments.
- Area and perimeter of shapes Rectangles, triangles, circles, trapezoids, ellipses and regular polygons.
- Volume of tanks and solids Boxes, cylinders, spheres, cones, truncated cones and pyramids, in cubic metres and litres.
- Triangle from three sides Area (Heron's formula) and angles of a triangle from its sides only: useful to survey land with a string.
- Right triangle (Pythagoras) Hypotenuse, legs and angles of a right triangle; check square corners with the 3-4-5 rule.
- Slope and height difference Convert between percent, degrees and 1:n ratio, and work out the drop of a ditch, a road or a roof.
- Height from shadows Height of a tree, building or tower by comparing its shadow with that of a vertical stick.
- Distance by pacing Turn paces into metres by calibrating your pace over a known distance.
- Map and plan scale Convert measurements from a map or plan to real life and back with 1:N scales.
Building and materials
Concrete and mortar mix ratios, bricks, stairs, weights, timber, roofing and excavation.
- Concrete and mortar mix design Cement, sand, gravel and water for a volume of concrete or mortar, by the dry-volume method.
- Bricks and mortar for a wall Number of bricks and approximate volume of bonding mortar to build a wall.
- Steps of a staircase Number of steps, a comfortable riser and tread by Blondel's rule, and the staircase's length.
- Weight of a volume of material Approximate mass of a volume of water, aggregate, timber, metal or other materials, from its typical bulk density.
- Volume of a round log Log volume by Huber's or Smalian's method, and mass if you know the density.
- Real area of a sloped roof Roof area from the plan area and the slope, and an approximate number of tiles.
- Excavation and soil bulking Volume of soil in the bank and once dug out (bulked), and transport trips based on the vehicle's capacity.
Water and food
Bleach chlorination, rainwater harvesting, tanks, rations, cropland area, planting layouts and brines.
- Bleach to disinfect water Millilitres and drops of bleach for a volume of water, resulting dose and minimum contact time.
- Rainwater harvesting Litres collected by a roof or ground surface from its area, the rainfall, the material and the first-flush diversion.
- Tank capacity and days of reserve Litres held by a cylindrical or rectangular tank and how many days it supplies a group of people.
- Water and food for a group Total water, energy and kilograms of a staple food to feed a group for several days.
- Cropland to feed a group Hectares of a crop needed to cover a group's energy for a year, allowing for losses and seed.
- Plants that fit in a plot Number of plants and seeds from the area, the spacing and a square/rectangular or triangular layout.
- Brine and salting Grams of salt for a brine of a given strength, with approximate total mass, density and volume.
Health
Support calculations for doses, rehydration, fluids, drip rates and body measurements. They do not replace clinical judgement: always check against the package leaflet or with a health professional.
- Medicine dose by body weight Milligrams per dose and per day, millilitres of syrup or number of tablets from a dose prescribed in mg/kg.
- Oral rehydration solution Sugar and salt for the home recipe, or ingredients of the WHO formula, for the litres you want to make, and the volume to replace in moderate dehydration.
- Maintenance fluids Baseline daily and hourly water requirements by body weight, using the Holliday-Segar rule (100-50-20 and 4-2-1).
- IV drip rate Drops per minute, seconds between drops and millilitres per hour to run a volume over a given time with a gravity drip set.
- Body mass index (BMI) An adult's BMI from weight and height, and the weight range that corresponds to a normal BMI at that height.
- Body surface area Body surface area in square metres using the Mosteller and DuBois formulas, from weight and height.
Energy, electricity and mechanics
Ohm's law, voltage drop, off-grid solar and hydro installations, wind turbines, transmissions, levers and firewood energy.
- Ohm's law and power Calculates voltage, current, resistance and power from two of them.
- Voltage drop in a cable Voltage drop, power lost and minimum cross-section of a cable based on its length, current and material.
- Off-grid photovoltaic solar installation Panel power and battery capacity needed based on daily consumption and days of autonomy.
- Power of a water head (micro-hydro) Hydraulic and electrical power of a water head based on flow rate and height.
- Power of a wind turbine Power available in the wind and useful power of a wind turbine based on rotor diameter and wind speed.
- Pulleys and gears Transmission ratio, output rpm and output torque in a pair of pulleys or gears.
- Levers and block and tackle Force needed to lift a load with a lever or a block and tackle, based on mechanical advantage and friction.
- Firewood energy Calorific value and useful energy of firewood based on its moisture content and stove efficiency.
Chemistry and workshop
Dilutions, solutions, lye for soap, temperature of water mixtures and alloy composition.
- Dilute a solution How much concentrated solution and how much water you need to lower a concentration (C1·V1 = C2·V2).
- Prepare a solution Grams of solute for a concentration in % w/v, % w/w, g/L or mol/L, corrected for reagent purity.
- Lye for soap making Grams of sodium or potassium hydroxide and of water to saponify up to three oils or fats, with superfat.
- Temperature of a water mixture Final temperature when mixing two amounts of water, or how much hot or cold water to add to reach a temperature.
- Alloy composition Kilograms of each metal to cast bronze, brass or soft solder, compensating for losses to oxidation and dross.
Arithmetic, time and navigation
Rule of three, percentages, travel times and orientation by the sun and stars without instruments.
- Rule of three Direct (proportional) or inverse rule of three from three known values.
- Percentages Calculate X% of a quantity, what percentage a part represents, or the variation between two values.
- Distance to a storm from the thunder Calculates how far away a lightning strike hit from the seconds between the flash and the thunder.
- Time, speed or distance of a trip Calculates the time, average speed or distance of a trip on foot or with any means of transport.
- Latitude from the sun or the North Star Calculates your approximate latitude from the sun's height at solar noon (measured directly or with the shadow of a stick) or from the height of the North Star.
- Day length by latitude and date Calculates how many hours of daylight there are at a place and date, and the approximate solar time of sunrise and sunset.