Blog
Guides built on the calculators: every figure is computed by the calculator it links to.
- Loans
How much interest do extra mortgage payments save?
Interest and time saved by adding $100 to $500 a month to a $300,000 mortgage at 5% over 25 years, computed with the so67 extra mortgage payment calculator.
- Salary
How much is $25 an hour per year?
Convert an hourly wage to an annual salary and back: $25 an hour is $52,000 a year on a 40-hour week, computed with the so67 hourly and annual salary calculators.
- Inflation
What will $1,000 be worth in 20 years?
Purchasing power of $1,000 after 10, 20 and 30 years of 2% to 4% inflation, computed with the so67 inflation calculator.
- Investing
How long does it take to double your money?
The rule of 72 against exact compound growth at 2% to 12%: years to double your money, and how far the shortcut drifts from the true figure.
- Credit cards
How long does it take to pay off a credit card?
A $5,000 balance at 19.99%: months and interest when you pay the minimum versus a fixed monthly amount from $100 to $500.
- Loans
Personal loan: how rate and term change the cost
A $15,000 personal loan at 6% to 15% over 36 and 60 months: monthly payment and total interest, computed with the so67 loan calculator.
- Health
Body fat percentage vs BMI: which to trust
How body fat is estimated from a BMI formula or the U.S. Navy tape-measure method, why the two can disagree and what each one misses.
- Health
Pregnancy due date: how it is calculated
How a due date is estimated from the last menstrual period or a known conception date, what 40 weeks means and why only a few babies arrive on the day.
- Health
How much water should you drink a day?
Reference daily water intake for adults, pregnancy and breastfeeding: total water, how much comes from drinks and how to add extra fluid lost.
- Travel
How much does a road trip cost in fuel?
Fuel cost of a road trip from distance, consumption per 100 km and fuel price, with a worked example, a table by distance and a way to split the bill.
- Home
How much does an appliance cost to run?
Electricity cost of an appliance per month and per year from its wattage, hours of use and price per kWh, with a worked example and a table by appliance.
- Money
How to calculate a tip and split the bill
Tip by bill size at 15%, 18% and 20%, and how to split a restaurant bill including the tip between several people, with a worked example.
- Loans
How much does a mortgage cost?
Monthly payment and total interest on a $300,000 mortgage at 4% to 7% over 20, 25 and 30 years, computed with the so67 mortgage calculator.
- Loans
Car loan: 36, 48, 60 or 72 months?
Monthly payment, total interest and total cost of a $30,000 car loan at 6% over 36, 48, 60 and 72 months.
- Investing
Compound interest: 10, 20 and 30 years
What $10,000 and $200 a month become at 7% a year after 10, 20 and 30 years, computed with the so67 calculators.
- Retirement
How much should you save for retirement?
Projected savings at 65 for $250, $500 and $1,000 a month over 15, 25 and 35 years at 5%, from the so67 retirement calculator.
- Health
BMI: formula, ranges and limits
How body mass index is calculated, the standard adult ranges, the healthy weight range by height and what BMI cannot tell you.
- Nutrition
Daily calories: the Mifflin-St Jeor method
How daily calorie needs are estimated: resting energy from the Mifflin-St Jeor equation, then an activity multiplier.
- Debt
Pay off debt faster: avalanche or snowball?
Avalanche (highest rate first) against snowball (smallest balance first), compared on three real-sized debts with $200 extra a month.
- Finance
Effective rate vs nominal rate
Why a 12% nominal rate costs 12.68% a year when interest compounds monthly, and how to compare offers fairly.
- Home
How much paint do you need for a room?
Paint volume for a room from wall perimeter, height, openings, coats and coverage, with a worked example and a table by room size.
- Math
Five everyday percentage calculations
Discounts, tips, sales tax, percentage change and shares of a total, each with one worked example.
- Conversions
Convert units without mistakes
The two conversions people get wrong most, pounds to kilograms and Celsius to Fahrenheit, with exact factors and a reference table.
- Method
How we check our numbers
How so67 calculators work, how the figures in blog articles are produced and what they cannot tell you.