A Plain-English Guide to Boomer Calc
Boomer Calc has four calculators: an everyday calculator, a mortgage payment calculator, a body mass index (BMI) calculator and a tip splitter. This guide explains what each one does, the formula behind it, and a worked example you can check with a pencil or with the calculator itself. The numbers in every example below are the numbers the calculator shows when you type them in.
The calculators run in your browser or on your phone. The numbers you type are not sent to us or saved.
1. The everyday calculator
The first tab is a standard calculator with large buttons. You can type a whole calculation, including parentheses, and press = to get the answer. It follows the usual order of operations: anything in parentheses first, then multiplication and division, then addition and subtraction, working left to right.
- C clears the display back to 0.
- * means multiply and / means divide.
- sin gives the sine of an angle measured in degrees (not radians), so sin(30) is 0.5 and sin(90) is 1.
- After an answer appears, pressing +, -, * or / carries on from that answer. Pressing a number starts a new calculation.
- Answers are shown to up to 12 significant digits.
Example: splitting the cost of a $12 and an $8 item three times over, then sharing it four ways.
Type (12+8)*3/4 and press =.
The calculator adds 12 + 8 = 20 first because of the parentheses, then 20 × 3 = 60, then 60 ÷ 4 = 15.
If something cannot be worked out, the calculator tells you why instead of showing a wrong number. Dividing by zero shows “Cannot divide by zero.” A missing closing parenthesis shows “Close every parenthesis.”
2. The mortgage payment calculator
The mortgage tab estimates the monthly payment on a fixed-rate home loan. You enter the home price, your down payment, the loan term (15, 20 or 30 years) and the annual interest rate. The loan amount is the home price minus the down payment.
The calculator uses the standard fixed-rate loan formula:
Monthly payment = P × r ÷ (1 − (1 + r)−n)
- P is the amount borrowed
- r is the monthly rate: the annual rate ÷ 12 ÷ 100
- n is the number of monthly payments: years × 12
If the rate is 0%, the payment is simply P ÷ n.
Example: a $300,000 home with $60,000 down, a 6.5% rate and a 30-year term.
- Amount borrowed: $300,000 − $60,000 = $240,000
- Monthly rate: 6.5 ÷ 12 ÷ 100 = 0.0054167
- Number of payments: 30 × 12 = 360
Estimated monthly payment: $1,516.96.
Over 360 payments that is about $546,100 in total, so roughly $306,100 of it is interest. The same loan over 15 years costs $2,090.66 a month, but the total interest drops to about $136,300.
What this number leaves out. The result covers principal and interest only. Most homeowners also pay property tax and homeowners insurance each month, often collected by the lender in an escrow account, and some pay mortgage insurance or homeowners association dues. Your real monthly bill is usually higher than this estimate. Use it to compare loans side by side, not as a quote. Your lender's Loan Estimate is the official figure.
A tip for refinancing or downsizing. If you are comparing a new loan with the one you have, enter your remaining balance as the home price and 0 as the down payment. That shows the payment on the balance you would actually be borrowing.
3. The BMI calculator
Body mass index compares your weight with your height. You can use pounds and inches or kilograms and centimetres. The calculator converts everything to kilograms and metres and then applies the standard formula:
BMI = weight in kilograms ÷ (height in metres × height in metres)
It uses the exact conversions 1 pound = 0.45359237 kg and 1 inch = 0.0254 metres.
Example: 150 pounds and 67 inches tall (5 feet 7 inches).
- Weight: 150 × 0.45359237 = 68.04 kg
- Height: 67 × 0.0254 = 1.7018 metres
- 1.7018 × 1.7018 = 2.896
- 68.04 ÷ 2.896 = 23.5
In metric, 68 kg and 170 cm also gives 23.5.
Boomer Calc shows the number only. For adults, the U.S. Centers for Disease Control and Prevention group BMI like this: under 18.5 is underweight, 18.5 to under 25 is a healthy weight, 25 to under 30 is overweight, and 30 or more is obesity.
BMI has limits, especially as we get older. It does not measure body fat directly and cannot tell muscle from fat. People tend to lose muscle and height with age, so the same BMI can mean something different at 70 than at 30. Treat the number as one rough indicator and talk to your doctor about what a healthy weight means for you. Boomer Calc is a calculator, not medical advice.
4. The tip calculator
Enter the bill, choose a tip percentage with the quick buttons (10%, 15%, 18%, 20% or 25%) or the slider (0% to 50%), and enter how many people are paying. The calculator shows the tip, the total and each person's share.
Example: an $84.50 dinner bill, an 18% tip, split three ways.
- Tip: $84.50 × 0.18 = $15.21
- Total: $84.50 + $15.21 = $99.71
- Each person: $99.71 ÷ 3 = $33.236…, shown as $33.24
Each share is rounded to the nearest cent, so three people paying $33.24 each comes to $99.72, one cent over. When a split does not come out even, someone may need to round up or down by a penny.
The tip is worked out on the amount you enter. If your bill already includes a service charge or gratuity (common for larger groups), check before adding another tip, and decide whether you want to tip on the amount before or after tax.
Making the screen easier to read
- iPhone and iPad: Settings → Display & Brightness → Text Size, or Settings → Accessibility → Zoom.
- Web browser on a computer: hold Ctrl (Command on a Mac) and press + to zoom in, or - to zoom out. Ctrl or Command and 0 resets it.
- Adding it to your home screen: in Safari on iPhone, tap the Share button, then “Add to Home Screen.” Boomer Calc then opens like an app.
Have a question that is not covered here? See the FAQ or email steveabramsdesigns@gmail.com.