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Percentages and date differences are two of the most common calculations people look up, and both are easy to get subtly wrong. A 20% increase followed by a 20% decrease does not return you to where you started. The gap between two dates in years, months, and days is not the same result you get by dividing total days by 30.

The calculators here handle these questions using clear formulas and calendar rules. The sections below explain how the calculations work so you can check the result rather than simply trust it.

Everything runs in your browser. ToolRino does not send, log, or store the numbers and dates you enter in these calculators.

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Percentage Calculator

Find percentages, proportions, and percentage changes.

Calculators

Age Calculator

Find the calendar years, months, and days between two dates.

Calculators

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Choose the right calculator

Pick the task you want to complete. Each tool includes instructions, an example, and its practical limits.

Percentage CalculatorChoose the calculation that matches your question before entering numbers. Finding 20% of 150 is different from asking what percent 20 is of 150. Percentage change compares a new value with a starting value; the tool uses the absolute magnitude of the starting value as its denominator.Age CalculatorFind the calendar duration between a birth date or starting date and another date. Select Use today for a current end date, or enter a historical date. Calendar months have different lengths, so the years-months-days result cannot be reproduced accurately by dividing a day count by a fixed 30-day month.

How to calculate percentages

The four common percentage questions

Most percentage problems fit one of these four patterns. The Percentage Calculator handles the first three directly; the fourth formula is included here for reference.

What is X% of Y?
Formula: (X ÷ 100) × Y
Example: 20% of 150 = (20 ÷ 100) × 150 = 30
Useful for: discounts, tips, tax, and commission.
X is what percent of Y?
Formula: (X ÷ Y) × 100
Example: 20 is what percent of 150? (20 ÷ 150) × 100 = 13.33%
Useful for: scores, shares of a total, and completion rates.
What is the percentage change from X to Y?
Formula: ((Y − X) ÷ |X|) × 100
Example: from 150 to 180 = ((180 − 150) ÷ 150) × 100 = 20% increase
Useful for: price changes, growth, and before-and-after comparisons.
What is Y after an X% increase or decrease?
Formula: increase Y × (1 + X ÷ 100); decrease Y × (1 − X ÷ 100)
Example: 150 after a 20% increase = 150 × 1.2 = 180
Useful for: markups, salary raises, and projected totals.

The mistake almost everyone makes

Percentage increases and decreases are not symmetrical because each is calculated against a different starting value. Take 100, add 20% to get 120, then subtract 20% from 120 to get 96. The increase was 20% of 100; the decrease was 20% of 120. This is also why a “50% off, then 20% off” sale gives a 60% total discount rather than 70%.

Percentages versus percentage points

If an interest rate moves from 4% to 6%, that is a rise of 2 percentage points and a 50% increase. Both statements describe the same change in different ways, and the distinction matters in finance and statistics.

Open the Percentage Calculator →

How to calculate age and date differences

Why calendar duration is not simple arithmetic

A duration expressed as years, months, and days depends on where in the calendar you start. Months range from 28 to 31 days, and leap years usually add a day every four years, except century years that are not divisible by 400.

Dividing a total day count by 30 or 365.25 produces an approximation rather than a calendar duration. The same number of elapsed days can be expressed differently in months and days depending on the start date.

How the calculation works

The calculator counts complete calendar months from the start date, expresses those as complete years and remaining months, and then counts the remaining days. When a matching day does not exist in a shorter month, it uses that month’s final day. This is a practical calendar convention; legal and organizational rules can differ.

What you can use it for

Age in years, months, and days for everyday forms and applications
Time elapsed since a past event or anniversary
Time remaining until a future deadline or exam
Length of employment, tenancy, or a subscription period
Gap between any two dates where the end is on or after the start

Open the Age Calculator →

Common calculator questions

How do I calculate percentage increase?

Subtract the original value from the new value, divide by the absolute value of the original, then multiply by 100. Going from 150 to 180 gives ((180 − 150) ÷ 150) × 100, which is a 20% increase.

What’s the difference between a percentage and a percentage point?

A percentage point is the arithmetic difference between two percentages. A percentage is a proportional change. A move from 4% to 6% is a rise of 2 percentage points and a 50% increase.

Why doesn’t a 20% discount cancel out a 20% increase?

Each is calculated against a different base. The increase applies to the original number, while the discount applies to the already increased number. Raising 100 by 20% gives 120; reducing 120 by 20% gives 96.

Does the age calculator account for leap years?

Yes. It uses actual calendar dates, so leap days and months of different lengths are included automatically. That result cannot be reproduced by dividing a day count by a fixed month length.

Can I calculate age for a future date?

Yes. The end date can be in the past or future as long as it is on or after the start date. Use today as the end date to calculate a current age.

Are my numbers sent to a server?

No. Both calculators run in your browser using JavaScript. ToolRino does not transmit, log, or store the values you enter in these calculators.

Do I need an account to use these calculators?

No. Every ToolRino calculator is free to use without signing up.

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