Free math calculators and converters

Percentages: Sales, Tips, Tax, and Change

Percentage problems come up constantly. You are shopping and see "30% off $85." You split a $124 dinner with a 20% tip four ways. Your revenue went from $47,000 to $52,000 and you need the growth rate. These are not math class problems — they cost you money if you get them wrong.

Worked example: A jacket costs $340. It is 15% off. To find the discount: multiply 340 × 0.15 = $51 off. Sale price: $340 − $51 = $289. Now add 8% sales tax: 289 × 0.08 = $23.12. Total you hand over: $289 + $23.12 = $312.12. The percentage calculator handles all three modes — X% of Y, X is what % of Y, and percentage change — so you skip the mental math on every purchase, raise calculation, or commission check.

Worked example: Your revenue went from $47,000 to $52,000. Percentage change: (52,000 − 47,000) / 47,000 × 100 = 10.64% growth. The percentage change calculator does this for any two numbers and tells you whether it is an increase or decrease. The discount calculator handles percent-off and fixed-amount discounts in one step. Use the tip calculator to split bills and tips across any number of people — accounts for tax and pre-tax tip too.

Grades: Where You Stand and What You Need on the Final

Grade anxiety is almost never about how you have done. It is about what you need next. You have an 82% in the class, the final is worth 20% of your grade, and you want at least an 85% to keep your A- average. How hard do you have to study?

Worked example: Current grade 82%, worth 80% of the total. Final exam worth 20%. Target final grade 85%. Required final exam score = (85 − 82 × 0.80) / 0.20 = (85 − 65.6) / 0.20 = 19.4 / 0.20 = 97%. You need a 97% on the final. If you cannot hit that, adjust the target. The grade calculator lets you change any variable — current grade, final weight, or target — until you find a realistic combination. The GPA calculator converts letter grades and percentages into a 4.0 scale with credit hours and weighted courses, so you see the full picture across every class this semester.

Conversions, Time Zones, and Dates

You follow a recipe calling for 250 mL but only have cups. A client in Singapore asks for a meeting at 9:00 SGT and you are in New York. A contract says "90 days from February 10" and you need the exact date. These are not complicated math problems, but they are easy to mess up under pressure.

The unit converter covers 8 categories — length, weight, temperature, volume, speed, area, energy, and pressure — so you can convert centimeters to inches, kilograms to pounds, Fahrenheit to Celsius, and more in one page. Need a specific conversion? The dedicated conversion pages cover 15 common pairs like inches to centimeters, miles to kilometers, and pounds to kilograms with precision up to 10 decimal places.

The time zone converter shows multiple time zones side by side, which is the only practical way to schedule across 3+ regions. The date calculator handles the two most common date problems: days between two dates (for deadlines, billing periods, age verification) and adding or subtracting days from a specific date (contract terms, subscription renewals). It also shows working days and calendar months so you do not accidentally land on a weekend.

Quick Measurements and Everyday Calculations

Biggest mistakes in home improvement come from guessing instead of measuring. A room that looks like 200 square feet is actually 192 sq ft (16 ft × 12 ft). That mistake means buying an extra box of flooring you cannot return — or worse, coming up short by one row. The square footage calculator handles rectangles, triangles, circles, L-shapes, and custom polygons with cost estimation so you buy the right amount of material the first time.

The age calculator gives exact age in years, months, and days along with next birthday countdown — useful for verification forms, benefits eligibility, or curiosity. The scientific calculator handles everything from basic arithmetic through trig, logarithms, and factorials with keyboard support and calculation history, for when you need more than a phone calculator but do not want to open a separate app.

Frequently Asked Questions

How do I calculate the percentage of a number?

Multiply the number by the percentage expressed as a decimal. To find 20% of $150: 150 × 0.20 = $30. For reverse problems — "X is what percent of Y" — divide X by Y and multiply by 100. $30 is what percent of $150? 30 / 150 × 100 = 20%. The percentage calculator handles both modes.

How do I convert centimeters to inches?

Divide centimeters by 2.54. Example: 50 cm / 2.54 = 19.69 inches. Going the other way: multiply inches by 2.54. Example: 12 in × 2.54 = 30.48 cm. The unit converter and dedicated conversion pages handle this and dozens of other conversions instantly.

How do I calculate what grade I need on the final?

Formula: (target grade − current grade × (1 − final weight)) / final weight. The result is what you need on the final expressed as a percentage. If your target is 90%, current is 85%, and the final is worth 25%: (90 − 85 × 0.75) / 0.25 = (90 − 63.75) / 0.25 = 105%. You cannot reach 90% — the final is not weighted high enough. The grade calculator shows you this instantly so you can adjust your target.

How do I calculate the number of days between two dates?

Count the days from the start date to the end date. The simplest way is to calculate the difference in total days, which accounts for months with different lengths. The date calculator does this automatically and also shows working days (excluding weekends) and the date in calendar months — useful for contract deadlines and billing cycles.

How do I calculate square footage of a room?

For a rectangle, multiply length by width. A room 15 ft by 12 ft = 180 sq ft. For triangles, multiply base by height and divide by 2. For circles, multiply the radius squared by π (3.1416). The square footage calculator handles all common shapes and includes cost estimation for flooring, paint, or materials.

Related categories

• Finance tools for freelancers
• Health & fitness calculators
• Freelancer business tools