Formulae in Everyday Life
What, where and which maths you actually use every day — from doubling a recipe to reading your electricity bill. 233+ real-world examples across 17 everyday categories, each linked straight to the formula behind it.
Cooking & Baking
16From doubling a recipe to nailing bread hydration — the maths behind every great meal.
Doubling or halving a recipe
Scale every ingredient up or down for a different number of servings without ruining the ratios.
Getting bread dough hydration right
Bakers set water as a percentage of flour weight to control how soft or crusty a loaf turns out.
Baker’s percentages for consistent loaves
Professional bakeries express every ingredient relative to flour so a recipe scales to any batch size.
Working out food cost per plate
Restaurants price a dish by adding up ingredient costs and comparing to the menu price.
Adjusting oven temperature (fan vs conventional)
Convert between fan-forced and conventional oven settings so bakes don’t burn or stay raw.
How long to roast a joint of meat
Estimate roasting time from the weight of the meat and the cut.
Making simple syrup for drinks
Bartenders mix sugar and water in a set ratio for cocktails and iced coffee.
Why water boils cooler up a mountain
At altitude water boils below 100°C, so cooking times change for pasta and eggs.
Counting calories in a meal
Add up calories from protein, carbs and fat to know the energy in what you eat.
Reading the % Daily Value on a label
Food labels show how much of your daily needs one serving provides.
Splitting your plate into macros
Work out what share of your calories comes from protein, carbs and fat.
Salting a brine for meat or pickles
Get the salt concentration right so food is seasoned and preserved, not ruined.
Converting a recipe from cups to grams
Scale ingredient quantities accurately when switching measuring systems.
Portion cost for meal prep
Divide the total cost of a batch by the number of servings you get.
How much raw meat to buy after trimming
Account for waste and shrinkage so you buy the right amount.
Sweetness of jam and syrups (Brix)
Measure sugar concentration so preserves set and keep.
Shopping & Deals
14Spot the real bargain, split the bill and never overpay at the till.
Working out a sale discount
See the actual price after ‘30% off’ before you get to the checkout.
Adding sales tax or VAT
Know the final price once tax is added to the sticker price.
Which pack is better value?
Compare price per litre or per kilo to find the cheapest option on the shelf.
Splitting a restaurant bill
Divide the total fairly between friends, tip included.
Calculating a fair tip
Work out 10%, 15% or 20% of the bill in your head.
Is the ‘20% extra free’ pack worth it?
Compare percentage change to judge bonus-size promotions.
Stacking two discounts
Apply a coupon on top of a sale to find the true final price.
Understanding markup vs margin
See how much a shop adds on top of cost to set the retail price.
Percentage off in your head
Quickly estimate what you save on any percentage discount.
Price rise since last year
Measure how much a product’s price has gone up or down.
Sales tax on a big purchase
Add the exact tax on furniture, electronics or a car.
Tip on a group dinner
Calculate a shared gratuity before splitting.
Loyalty points to cash value
Convert points and rewards into real money saved using ratios.
Buy-one-get-one deals
Work out the effective per-item price of BOGOF offers.
Money & Banking
16Loans, savings, interest and inflation — the numbers that run your wallet.
Monthly repayment on a loan
Work out the fixed monthly instalment on a car or personal loan.
How savings grow with compound interest
See how money snowballs when interest earns interest.
Simple interest on a short loan
Calculate flat interest on a deposit or short-term borrowing.
The Rule of 72 — doubling your money
Estimate how many years it takes savings to double.
What a future goal is worth today
Discount a future sum back to today’s money (present value).
How much a lump sum becomes
Project the future value of money you invest now.
Return on an investment
Measure the percentage gain or loss on money you put in.
Average yearly growth of an investment
Compare funds using the compound annual growth rate.
What inflation does to your cash
See how rising prices erode the buying power of savings.
Adjusting an old price for inflation
Compare prices across years in real terms.
Effective vs advertised interest rate
Find the true annual rate once compounding is included.
Total interest paid over a loan
Add up everything a loan costs beyond the amount borrowed.
Converting money on holiday
Turn your home currency into the local one at the exchange rate.
Why the reverse rate looks different
Understand the inverse exchange rate when converting back.
Depreciation of a car or laptop
Track how an asset loses value each year.
Saving regularly into a fund
See what monthly contributions grow to (annuity future value).
Home & DIY
14Paint, tiles, flooring and furniture — measure once, buy the right amount.
How much paint to buy
Work out litres of paint from wall area and coverage per tin.
Counting tiles for a floor or wall
Estimate how many tiles cover a room, including spares.
Area of a room
Measure floor area for carpet, laminate or rugs.
Area of a rectangular wall
Get the square metres before ordering wallpaper or paint.
Will the sofa fit through the door?
Use the diagonal of a rectangle to check clearances.
Volume of a fish tank or planter
Find the litres a box-shaped container holds.
Concrete for a cylindrical post hole
Work out the volume of a cylinder of concrete.
Fencing or skirting length
Add up the perimeter of a room or garden.
Cutting a diagonal brace
Use Pythagoras to get the length of a diagonal support.
Area of a triangular gable
Measure an odd triangular wall section for paint.
Water in a round pool
Find the volume of a cylindrical paddling pool.
Carpet for an L-shaped room
Break the floor into rectangles and add the areas.
Sloping a wheelchair ramp or drain
Work out gradient so a ramp or pipe drains correctly.
Mixing cleaning solution or feed
Dilute a concentrate to the right strength.
Driving & Cars
15Fuel, speed, stopping distance and range — the maths behind the wheel.
Your car’s fuel economy
Work out miles per gallon or litres per 100 km from a tank.
Cost of a road trip in fuel
Estimate what a journey costs at the pump.
Arrival time on a journey
Work out your ETA from distance and average speed.
Average speed on a trip
Divide distance by time to find your average pace.
Safe braking distance
See how far a car travels before it stops at a given speed.
0–60 mph estimate
Estimate acceleration time from power and weight.
Engine power vs weight
Compare cars using their power-to-weight ratio.
Miles per gallon
Turn fuel used and distance into MPG.
Gear ratio and wheel speed
See how gearing turns engine revs into road speed.
Electric car range
Estimate how far an EV goes on a full battery.
EV charging time
Work out how long a charge takes from battery size and charger power.
Engine displacement
Understand the ‘2.0 litre’ figure from bore and stroke.
Horsepower from torque and revs
Convert torque and RPM into horsepower.
Tyre size and revolutions
Work out how far one tyre revolution takes you.
km/h to mph while abroad
Convert speed-limit signs to units you know.
Health & Body
15BMI, calories, hydration and heart rate — numbers that keep you well.
Your Body Mass Index
Check whether your weight is healthy for your height.
Calories you burn at rest (BMR)
Find the energy your body uses just to stay alive.
Daily calories you need (TDEE)
Work out total energy needs including activity.
Your maximum heart rate
Estimate the ceiling for safe exercise intensity.
Fat-burning heart-rate zone
Find the target range for effective cardio.
How much water to drink
Estimate daily hydration from body weight.
Calories burned in a workout
Turn exercise time and intensity into calories.
A healthy weight for your height
Estimate an ideal body-weight range.
Daily protein target
Work out grams of protein for your goals.
Calorie deficit to lose weight
See the daily gap needed to drop body fat.
Waist-to-hip ratio
A quick health marker beyond the scales.
Body-fat estimate
Estimate body-fat percentage from measurements.
Blood pressure (mean arterial)
Combine systolic and diastolic into one figure.
Pregnancy due date
Estimate the delivery date from the last period.
Converting a fever to °F/°C
Read a thermometer in the units you know.
Fitness & Sports
14Running pace, lifting maxes and the stats behind the game.
Your running pace
Turn distance and time into minutes per km or mile.
One-rep max on a lift
Estimate your max from a lighter set of reps.
Race finish-time prediction
Use pace to project your marathon or 10k time.
How far a ball travels
Projectile range of a kicked or thrown ball.
Peak height of a jump or shot
Maximum height of a projectile.
Hang time of a punt
How long a ball stays in the air.
Expected goals (xG) in football
The stat behind whether a chance should have scored.
Elo rating in chess or esports
How player rankings rise and fall after games.
Batting average in cricket
Runs per dismissal, the classic batting stat.
Strike rate in cricket
How quickly a batter scores per 100 balls.
Golf handicap
Level the field so any two golfers can compete.
Cycling power output
Watts you push on the pedals.
Win probability (Pythagorean)
Predict a team’s wins from points for and against.
VO2 max fitness score
Estimate aerobic fitness.
Weather & Nature
14‘Feels like’ temperatures, storms and the science of the sky.
‘Feels like’ heat on a humid day
The apparent temperature combining heat and humidity.
Wind chill in winter
How much colder wind makes it feel.
Dew point and morning mist
The temperature at which dew and fog form.
Relative humidity
How saturated the air is with moisture.
Reading wind on the Beaufort scale
Turn wind speed into a describable force.
Force of wind on a wall or sign
Wind pressure on a flat surface.
Heat index warning
Combine temperature and humidity for a safety index.
How strong was that earthquake?
Read the moment-magnitude scale.
Speed of a tsunami wave
How fast a tsunami crosses shallow water.
‘Once in 100 years’ flood
What a flood return period really means.
Air pressure and altitude
How pressure falls as you climb.
Temperature drop with height
Why mountaintops are colder (lapse rate).
Rainfall runoff from a storm
Estimate peak runoff for drains (rational method).
Wind farm power
How much electricity a wind turbine makes.
Tech & Devices
14Storage, downloads, screens and the numbers inside your gadgets.
How long a download takes
Turn file size and connection speed into a time.
Bandwidth for streaming
Work out the data rate a video needs.
GB, MB and file sizes
Convert between storage units.
Bits needed to store a value
How many bits represent a number or colour.
Binary to decimal
Read the ones and zeros computers use.
Hex colour codes
Convert hex to decimal for web colours.
Screen aspect ratio
Work out 16:9 vs 4:3 for photos and video.
Megapixels on a camera phone
How resolution turns into megapixels.
Data cap and monthly usage
Track whether streaming will blow your data plan.
Wi-Fi channel capacity
The Shannon limit on how much data a channel carries.
Password strength
How hard a password is to crack (entropy).
Battery charge time
Estimate how long a device takes to charge.
LED resistor for a hobby circuit
Pick the right resistor so an LED doesn’t burn out.
Sampling rate for audio
The Nyquist rate for digitising sound.
Photography & Media
12Aperture, depth of field and resolution for sharper shots.
Aperture and f-number
How the f-stop controls light and blur.
Depth of field
How much of a scene is in focus.
Hyperfocal distance
Focus point for maximum sharpness front to back.
Field of view of a lens
How wide a shot a focal length gives.
Crop factor on smaller sensors
Why a lens looks more zoomed on a crop camera.
Exposure and the sunny-16 rule
Balance shutter, aperture and ISO.
Megapixels and print size
How big you can print at good quality.
Print resolution (PPI)
Pixels per inch for a crisp print.
Motion blur from shutter speed
Whether a moving subject blurs.
Flash guide number
How far a flash reaches at a given aperture.
Video aspect ratio
Frame for 16:9 or vertical 9:16.
Ground resolution for drone maps
How much ground each pixel covers.
Music & Sound
12Tuning, tempo, delay and the physics of what you hear.
Frequency of a musical note
The pitch in hertz of any note.
Setting a delay pedal to the beat
Delay time in ms that matches the tempo.
How loud is loud? (decibels)
Understand the decibel scale of sound.
Adding two sound sources
Why two speakers aren’t twice as loud.
Tuning octaves
Why an octave up doubles the frequency.
Cents and fine tuning
Measure tiny pitch differences.
Guitar string pitch
How length and tension set the note.
Tempo to beat length
Turn BPM into seconds per beat.
Harmonics and overtones
The series that gives an instrument its tone.
Room reverb time
How long sound lingers in a room (Sabine).
Doppler shift of a siren
Why a passing siren changes pitch.
Wavelength of a note
How long a sound wave is in air.
Work & Business
15Pricing, profit, break-even and the KPIs that run a business.
Break-even point
How many sales you need to cover costs.
Profit margin
What percentage of revenue you keep.
Setting a price with markup
Add margin on top of cost to set a price.
Return on investment
Measure the payoff of a business spend.
Monthly recurring revenue
The core SaaS subscription metric.
Customer lifetime value
How much a customer is worth over time.
Cost to acquire a customer
What you spend to win each new customer.
Churn rate
How fast customers are leaving.
Conversion rate
Share of visitors who buy.
Economic order quantity
The cheapest amount of stock to reorder.
Gross profit
Revenue minus cost of goods sold.
Net profit margin
The bottom-line profitability percentage.
Advertising return (ROAS)
Revenue earned per dollar of ad spend.
Click-through rate
How often an ad or link gets clicked.
Inventory turnover
How quickly stock sells and is replaced.
Garden & Outdoors
12Seed, feed, water and grow — the maths of a healthy plot.
How much seed for a lawn or field
Seed rate per square metre or hectare.
Fertiliser to spread
Work out how much feed a plot needs.
Reading an NPK label
Balance nitrogen, phosphorus and potassium.
Water a crop needs
Estimate irrigation from evapotranspiration.
Plant spacing and population
How many plants fit in a bed.
Expected yield
Estimate harvest from area and rate.
Seed germination rate
How many seeds will actually sprout.
Soil moisture
Check how wet the soil is.
Compost or feed dilution
Mix a liquid feed to the right strength.
Area of a garden bed
Measure a plot for turf or mulch.
Volume of soil or mulch
Cubic metres to fill a raised bed.
Water in a rain barrel
Volume a cylindrical butt holds.
Home Energy & Bills
12Electricity, heating and solar — understand and cut your bills.
Cost of running an appliance
Turn watts and hours into kWh and money.
Power a device draws
Watts from volts and amps.
Solar panel output
How much electricity panels generate.
Your carbon footprint
Estimate emissions from energy use.
Home energy consumption
Add up total household energy use.
Air-conditioner efficiency (EER)
Compare cooling units by efficiency.
Heat pump performance (COP)
How much heat you get per unit of power.
Sizing an AC for a room
Cooling tons or BTUs a room needs.
Heat lost through a wall
How insulation (R-value) cuts heat loss.
Fridge efficiency
The coefficient of performance of a fridge.
kWh to joules
Convert energy units on your bill.
Heating degree days
Estimate seasonal heating demand.
Odds & Chance
12Lotteries, games and risk — what the odds really are.
Chance of a single event
Basic probability of something happening.
Lottery combinations
How many ticket combinations exist.
Ways to arrange things
Permutations for passwords or line-ups.
Odds vs probability
Convert ‘5 to 1’ into a percentage.
Expected value of a bet
The average outcome over many plays.
Chance of at least one win
Complement rule for ‘at least one’.
Shared birthday surprise
Why a room shares a birthday sooner than you think.
Rolling two dice
Combine independent probabilities.
Either-or outcomes
Addition rule for combined events.
Conditional chance
Probability given something already happened.
Bayes’ rule and false positives
Why a positive test can still be unlikely.
Streaks and coin flips
Binomial chance of runs of heads.
Sky & Space
12Stargazing curiosities — distances, orbits and light from the cosmos.
How far is a light-year?
Turn light-travel-time into a distance.
Escape velocity from a planet
The speed to break free of gravity.
Speed a satellite orbits
Orbital velocity of the ISS or a moon.
How long a planet’s year is
Kepler’s law linking orbit size and period.
Brightness of a star
Apparent vs absolute magnitude.
Distance from parallax
How astronomers measure star distances.
Gravity on another world
Your weight on the Moon or Mars.
Redshift of a galaxy
How we know the universe is expanding.
Angular size of the Moon
Why the Moon and Sun look the same size.
Age of the universe (Hubble)
Estimate cosmic age from expansion.
A black hole’s size
Schwarzschild radius from mass.
Parsecs and star distance
The unit astronomers actually use.