Everyday Life Calculators

A Guide to Everyday Life Calculators

The calculations in this category are the ones nobody teaches and everybody needs: splitting a bill fairly when one person did not drink, working out whether a subscription is worth keeping, deciding whether driving somewhere costs more than the train. Small sums, repeated often, that add up to real money over a year.

Thirty-seven calculators here, grouped by where they come up.

Household bills

Five calculators for the costs that arrive whether you think about them or not. The Electricity Cost Calculator works from your tariff and usage, and the Appliance Energy Calculator breaks that total down by device, which is the only way to find out what is actually expensive. The answers surprise people: a standby load of a few watts running continuously costs more over a year than an appliance that draws a kilowatt for twenty minutes a week.

The Gas Bill Calculator handles the unit conversions energy suppliers use, which differ between meters and between countries. The Water Usage Calculator estimates consumption by activity, and the Laundry Cost Calculator totals the electricity, water and detergent for a load, where the heating of the water dominates everything else.

Driving and transport

Six calculators. The MPG Calculator and Fuel Consumption Calculator measure what your car actually does rather than what the manufacturer claimed, using odometer readings and a fill.

From there, the Fuel Cost Calculator prices a journey, the Commute Cost Calculator annualises the daily version, and the Road Trip Calculator plans fuel, time and stops for a long drive. The EV Charging Cost Calculator covers the electric equivalent, where the cost depends enormously on whether you charge at home or on a public rapid charger.

The commute figure is the one worth running once. A short daily drive priced per journey looks trivial and priced per year rarely does.

Eating and drinking out

Four calculators. The Tip Calculator handles the percentage and the rounding, and the Bill Split Calculator divides a bill that was not consumed equally, which is the version people actually need and the one most tip calculators cannot do.

The Pizza Size Calculator compares value by area rather than diameter, and the result is routinely counterintuitive: an 18-inch pizza has more than twice the area of a 12-inch, because area goes with the square of the diameter. Two mediums are usually worse value than one large. The Party Drinks Calculator estimates quantities by guest count and duration.

Shopping and spending

Five calculators for deciding whether something is worth it. The Cost Per Use Calculator is the most useful idea in this category: it converts a purchase price into a cost per wear or per use, which is how an expensive item worn weekly beats a cheap one worn twice.

The Shopping Savings Calculator and Coupon Stacking Calculator work out what a combination of offers actually comes to, where the order of application changes the total. The Subscription Cost Calculator annualises recurring charges, which is the figure subscription pricing is designed to keep you from seeing. The Phone Plan Comparison Calculator compares plans on your own usage rather than on headline allowances.

Running a household

Three calculators for shared and recurring costs. The Rent Split Calculator divides rent between rooms of different sizes and amenities rather than equally, which is the fairer and far more argued-about version. The Grocery Budget Calculator sizes a weekly shop by household, and the Moving Cost Calculator estimates a move by volume and distance.

Events and gifts

The Wedding Budget Calculator allocates a total across the usual categories, which is more useful than a sum because the arguing is about proportions. The Gift Budget Calculator plans across a list of people and occasions.

Pets

Three calculators. The Dog Age Calculator and Cat Age Calculator both reject the multiply-by-seven rule, which is wrong at every point of a dog's life. The first year is worth far more than seven human years and later years far fewer, and the rate differs by breed size, so a small dog and a large one of the same age are not at the same life stage at all.

The Pet Food Calculator works out portions from weight and activity, which matters because the feeding guides printed on packaging are generous.

Aquariums

Four calculators, and two of them answer genuinely different questions despite similar names. The Fish Tank Stocking Calculator works from one adult fish size using the inch-per-gallon and surface-area rules, which is what you want when planning a single-species tank. The Aquarium Stocking Calculator handles a mixed community, weighting species by waste output and checking it against your filter's flow rate, which the simple rule cannot do.

The Aquarium Gravel Calculator and Aquarium Substrate Calculator work out the quantity for a given depth, the substrate calculator covering the planted-tank layers where the depth requirement is larger.

Crafts and seasonal

Three calculators that are hard to find elsewhere. The Candle Wax Calculator works from container volume to wax weight, accounting for the density difference between wax types. The Wrapping Paper Calculator sizes a sheet for a box, including the overlap. The Christmas Tree Light Calculator gives a length from tree height and the density you want, which is more lights than most people buy.

Home and garden

The Lawn Mowing Time Calculator estimates from area, mower width and walking speed, with the overlap that real mowing requires. The TV Size Distance Calculator works in both directions, either the screen size for your room or the viewing distance for a screen you already own, based on viewing angle rather than a single rule of thumb.

The three that save the most money

If you only run three of these, run these three.

The appliance energy calculator, because household electricity is the bill people most assume is fixed. It is not, and the distribution of cost across devices is lopsided enough that finding the top two or three usually finds most of the savings available.

The subscription cost calculator, because recurring charges are the easiest spending to forget and the hardest to notice. A handful of small monthly services that each feel negligible add up to a figure most people are surprised by when they finally total it, and nothing prompts you to total it.

The commute cost calculator, because a daily journey is priced in your head per trip and paid for per year. Fuel, wear and parking summed over a working year is the number that makes a different arrangement worth considering, and it is invisible until you calculate it.

Each of those three converts a cost you are already paying into a form you can act on, which is a better return than any of the one-off estimates here.

The pattern worth taking away

Nearly every calculator in this category does one of two things: it annualises something quoted per use, or it converts a headline price into a cost per unit of what you actually get.

Both exist because pricing is usually presented in whichever unit makes it look smallest, and that is a choice made by whoever is selling, not an accident of convention. A subscription is monthly, a commute is per day, a pizza is by diameter rather than area, a pet food bag is by weight rather than by servings. Converting to a comparable unit is the whole trick, and it changes decisions more often than you would expect.

None of this requires discipline or budgeting software. It requires doing each sum once, writing the answer down, and letting it inform the next decision of that kind. A cost per use worked out once tells you something about every similar purchase afterwards.