数学・幾何

Calculate the number of days between two specified dates quickly and accurately.

Estimate doubling time from a growth rate, or implied rate to double in N years. Educational mental math only.

Weighted grades: what you need on the final to reach a target course grade.

Enter grade points and credits per line; computes weighted GPA on a 4.0-style scale.

Target annual income, billable weeks/hours, and buffer % to a suggested hourly rate.

Solve a:b = c:x for x (cross-multiply: x = b×c÷a).

Integer range, how many numbers, optional unique draws (≤ range size).

Convert Arabic numbers to Roman numerals and parse common Roman strings back.

Total bill, headcount, and optional tip percent—per person share.

Radius → area πr² and circumference 2πr in consistent units.

Two legs of a right triangle—hypotenuse and area.

Principal square root for non-negative real x.

Air temperature °C and RH%—approximate dew point for planning.

Character pool size and length—approximate bits of entropy.

Greatest common divisor and least common multiple of two integers.

Physical length in cm and DPI—pixel count along that edge.

Small factorials for combinatorics homework checks.

Binomial coefficient C(n,r) with a stable multiplicative formula.

Given long or short side, find the other for φ ≈ 1.618 proportions.

Length × width × height in consistent units.

π r² h for a right circular cylinder.

(4/3) π r³ from radius.

Two (x,y) pairs—slope m (vertical line excluded).

Average of coordinates—center between two points.

Binary string of 0s and 1s to a decimal integer.

Non-negative integers up to about 2²⁰ to binary text.

Hex string (optional 0x) to a decimal integer.

Non-negative integers up to 2³²−1 to uppercase hex.

Base-8 digit string to a decimal integer.

Non-negative integers up to 2³¹−1 to octal text.

Common logarithm for positive x.

Exponent x—ten to the x power (limited range).

Diagonal inches (16∶9)—rough min–max distance in feet.

Up to three insulation layers—approximate total R.

Ordered arrangements—n choose r with ordering.

Add two fractions and reduce to lowest terms.

Value, mean, and standard deviation—standardize a score.

Length and charset—tokens and test IDs (not crypto secrets).

Radius and height—lateral and total surface area.

UTF-16 units vs Unicode code points.

Whitespace word count.

Three sides → triangle area.

Intensity ratio vs distance change.

Parse #RGB / #RRGGBB.

√(a²+b²) for a right triangle.

π times semi-major times semi-minor.

πr²h for a right circular cylinder.

4πr² from radius.

Fraction of circumference from central angle.

Pie-slice area from r and θ°.

180° minus two known angles.

Same terminal side in [0,360).

Euclidean distance in the plane.

Binary log for positive x.

Natural log for positive x.

Two-number harmonic average.

Each interior angle for regular n-gon.

Rule of three: direct, inverse, and percent proportions — multilingual (German default, English and more).