Move the decimal one spot left to get 10%, then adjust from there. For a $48 bill, 10% is $4.80. Want 20%? Double it to $9.60. Need 15%? Take half of 10% ($2.40) and add it to 10% ($4.80) for $7.20. The tip calculator on this site does it all automatically, but mental math is handy when you want to check.

How do you calculate BMI if you only know pounds and inches?

The formula is BMI = (weight in lbs × 703) ÷ (height in inches²). A 160-pound person who is 5'6" (66 inches): 160 × 703 = 112,480 divided by 66² (4,356) equals 25.8 — which falls in the overweight range. Or you can just plug the numbers into the BMI calculator above and get the answer in under a second.

What's the difference between a fixed and variable interest rate on a loan?

A fixed rate stays the same for the entire loan term — your monthly payment never changes. A variable rate can go up or down with the market, so your payment might increase later. Fixed rates usually start a little higher but give you predictability. Variable rates can save money if rates stay low, but they're a bet. The loan and mortgage calculators on this site work for both types.

How does compound interest actually grow your money over 30 years?

Compound interest means you earn returns on your returns. If you invest $10,000 at 7% and leave it alone for 30 years, it grows to about $76,000 — even though you only put in $10,000. The last few years do most of the work. That's why starting early matters more than how much you put in. The compound interest calculator on this site shows the year-by-year breakdown.

How many days are there between two dates if you count both start and end?

If you include both the start and end date, add 1 to the normal day count. January 1 to January 5 is normally 4 days apart, but if you count both days it's 5 days. The days-between-dates calculator has a toggle for this because different situations need different counting methods — like billing, legal deadlines, or event planning.

How do I figure out what grade I need on my final exam to pass a class?

Take your current grade as a decimal (like 0.82 for 82%), subtract it from your target grade (0.90 for an A-minus), then divide by your final's weight. If your final is worth 20% of the grade and you want a 90%: (0.90 - 0.82 × 0.80) ÷ 0.20 = 0.82, so you'd need about an 82% on the final. The grade calculator does this math instantly.

How much more house can I afford if I put 20% down vs. 5% down?

A bigger down payment means a smaller loan, which means lower monthly payments and less interest over time. On a $400,000 house: 5% down leaves a $380,000 loan, while 20% down cuts it to $320,000. At 6.5% interest, that difference saves roughly $500 per month and about $70,000 in total interest. The mortgage calculator shows both scenarios side by side.

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