See the math.

Start here

• Aquarium & reefAquarium filter flow & turnover calculatorWhat filter flow (GPH) does your aquarium need? Enter tank size and type to get a target turnover, and check your filter against it.
• Aquarium & reefAquarium heater size calculatorHow many watts does your aquarium heater need? Enter tank size and room temperature to get a wattage range and a heater to buy.
• Aquarium & reefReef alkalinity, calcium & magnesium dosing calculatorHow much baking soda, soda ash, calcium chloride, magnesium salt or Red Sea Foundation to raise reef alkalinity, calcium or magnesium.
• Aquarium & reefSalinity & specific gravity calculatorConvert hydrometer specific gravity to salinity in ppt, corrected for water temperature and calibration, and see how far you are from target.
• Aquarium & reefAquarium sand & substrate calculatorHow many pounds of sand, gravel or substrate does your tank need? Enter tank size and bed depth to get the weight to buy, with a diagram.
• Aquarium & reefAquarium volume calculatorHow many gallons is your fish tank? Rectangle, cube, cylinder, bow-front, hexagon and corner tanks, plus the real water left after sand and headroom.
• Aquarium & reefWater change & nitrate calculatorHow much water to change to bring nitrate down to your target, plus a chart of nitrate over repeated water changes on your schedule.
• Aquarium & reefAquarium weight & stand load calculatorHow much does your filled fish tank weigh? Water, glass, substrate and rock, plus how much of your stand's rated capacity it uses.
• Off-grid powerBattery bank size calculatorHow many amp-hours of battery you need for an RV, van, boat or cabin, from your daily watt-hours and days of autonomy, with a 24-hour state-of-charge chart.
• Off-grid powerAmp-hour and battery runtime calculatorHow long a battery runs a load. Enter amp-hours, voltage, battery type and watts. Covers LiFePO4 and AGM, inverter losses and lead-acid discharge-rate loss.
• Off-grid powerSolar charge controller size calculatorWhat size solar charge controller you need, MPPT or PWM: its amp rating, the voltage it must take on a cold morning and the PV fuse, from your panels.
• Economics StudioConsumer and producer surplus calculatorFind consumer surplus, producer surplus and total surplus from straight-line demand and supply equations, with both triangles shaded on a graph you can drag.
• Travel & moneyCost of living comparison calculatorCompare the cost of living between any two US states or metro areas using BEA price data, with the salary you need to keep the same standard of living.
• Off-grid powerDC wire size and voltage drop calculatorFind the AWG wire size for a 12, 24 or 48 V DC circuit from current and distance. Checks voltage drop and ampacity with NEC and Coast Guard tables.
• Travel & moneyTravel data calculatorHow much eSIM data do you need for your trip? Add your daily maps, social media, video and calls to get the GB to buy and the plan size that covers it.
• Economics StudioGDP deflator calculator (real vs nominal GDP)Find real GDP from nominal GDP and the deflator, the deflator from nominal and real GDP, or real growth and inflation over two years or from a basket of goods.
• Pool, hot tub & pondHot tub chemical calculatorHow much dichlor or chlorine, baking soda, pH down and calcium to balance your hot tub by its size, in order, with before and after meters for spa ranges.
• Pool, hot tub & pondHot tub volume calculatorEstimate how many gallons your hot tub or spa holds from its size, for portable acrylic spas, wooden and in-ground tubs, and inflatables.
• Off-grid powerInverter size calculatorWhat size inverter you need for a van, RV or cabin. Continuous watts for what runs together, surge watts for the biggest motor starting, and battery current.
• Economics StudioMoney multiplier calculatorFind the money multiplier and how far a deposit or new reserves can grow the money supply, with bars for each round of lending and the banks’ balance sheet.
• Pool, hot tub & pondPond liner size & volume calculatorWhat size pond liner you need and how many gallons your pond holds, for rectangular, round, oval and free-form ponds, drawn on the liner.
• Pool, hot tub & pondPool chemical calculatorHow much chlorine, baking soda, acid, calcium chloride and stabilizer to balance your pool, in order, with before and after gauges for FC, pH, TA, CH and CYA.
• Pool, hot tub & pondPool heater size & heat-up cost calculatorWhat size pool heater you need in BTU, how long yours takes to heat the pool and what heat-up costs with gas, propane or a heat pump, with a heat-up curve.
• Pool, hot tub & pondPool salt calculatorHow many pounds of salt and how many bags take your saltwater pool from its current ppm to the level your chlorine generator wants, with a ppm meter and bags.
• Pool, hot tub & pondPool shock calculatorHow much liquid chlorine, bleach, cal-hypo or dichlor shocks your pool, with the shock (SLAM) level for your cyanuric acid and a free chlorine vs CYA chart.
• Pool, hot tub & pondPool volume calculatorHow many gallons is your pool? Rectangle, round, oval, oblong or kidney, with a flat or sloped floor, drawn from above and from the side.
• Off-grid powerPower station runtime calculatorHow long a portable power station runs your fridge and other appliances, and whether it can start them. Runtime bars for each appliance and for all together.
• Economics StudioProduction possibilities frontier (PPF) and opportunity cost calculatorDraw a production possibilities frontier, drag or type two points on it and see the opportunity cost of moving between them, with points inside or outside.
• Economics StudioPrice ceiling and price floor calculatorSee the shortage a price ceiling causes or the surplus a price floor leaves. Set a legal price on straight-line demand and supply and see who gains and loses.
• Economics StudioPrice elasticity of demand calculatorFind price elasticity of demand with the midpoint method between two points, or the point method on a straight demand line, and see it change along the curve.
• Off-grid powerSeries and parallel wiring calculatorWire batteries or solar panels in series, parallel or both and see the voltage, capacity and current you get, with a wiring diagram that updates as you type.
• Off-grid powerSolar panel size calculatorHow many watts of solar panels you need for an RV, van, boat or cabin from daily watt-hours and peak sun hours, with a chart of panel output against load.
• Off-grid powerOff-grid solar system builderList your loads and size the whole off-grid system at once, with battery amp-hours, solar watts, inverter size, main cable gauge and a 24-hour charge chart.
• Economics StudioSupply and demand studioDrag demand and supply curves to find equilibrium price and quantity, add a tax or subsidy, and see consumer surplus, producer surplus and deadweight loss.
• SurfSurfboard volume calculatorHow many litres should your surfboard be? Your volume range by weight, skill and board type, a board drawn to scale, and a check on any board you own.
• Economics StudioTax incidence and deadweight loss calculatorSee who really pays a per-unit tax. The tax wedge splits into buyers' and sellers' shares, with tax revenue and deadweight loss worked out and shaded.
• Travel & moneyTime zone converter & meeting plannerConvert a time between cities and see a 24-hour timeline of who is at work, awake or asleep, with the shared working hours for up to four places.
• SurfWetsuit thickness by water temperatureWhat wetsuit thickness do you need? Enter the water temperature for the suit to buy (3/2, 4/3, 5/4 or 6/5) and whether you need boots, gloves or a hood.

Topics

• Aquarium & reefTank weight, heaters, volume, dosing and more.
• Pool, hot tub & pondVolume, chemicals, salt and heating.
• Off-grid powerSolar, batteries, inverters and wire sized for real loads.
• Economics StudioDraggable supply and demand, elasticity, surplus, taxes, the PPF, GDP and the money multiplier.
• SurfSurfboard volume and wetsuit thickness for where and how you surf.
• Travel & moneyCost of living, time zones and travel data.