Calculator Guides and Formulas
Use these guides to understand formulas, statistics concepts, percentage problems, health calculations, and related calculators. Each guide is written to help you understand the steps behind the answer.
Latest
Browse the library
Twenty-one articles so far. We add more only when we can write them as carefully as the calculators themselves.
What Is Gematria? A Simple Guide to How Gematria Works
Plain-language explanation of gematria — what it is, how letter-to-number ciphers work in English, Hebrew, and Greek, and what the numbers can and can't tell you.
How to Calculate TDEE
Learn how to calculate TDEE using BMR and activity level, what the formula means, and how to use your result for weight loss, maintenance, or muscle gain.
BMR vs TDEE
Learn the difference between BMR and TDEE, how each one is calculated, and which number matters more for weight loss, maintenance, or muscle gain.
What Is TDEE?
Learn what TDEE means, what it includes, how it differs from BMR, and how to use your estimated daily calories for weight loss, maintenance, or muscle gain.
What Is Standard Deviation?
Learn what standard deviation means, what it tells you, how to interpret it, and how it relates to variance, averages, and spread in a data set.
Standard Deviation Formula
Learn the standard deviation formula, the difference between sample and population standard deviation, and how to calculate standard deviation step by step.
Variance vs Standard Deviation
Learn the difference between variance and standard deviation, how they are related, and when to use each one to understand spread in a data set.
How to Calculate Standard Deviation in Excel
Learn how to calculate standard deviation in Excel using STDEV.S and STDEV.P, when to use each formula, and how to check your result with a calculator.
Sample vs Population Standard Deviation
Learn the difference between sample and population standard deviation, when to use each formula, and how they relate to STDEV.S and STDEV.P in Excel.
Standard Deviation Symbol
Learn the standard deviation symbols used in statistics, including σ for population standard deviation, s for sample standard deviation, μ, x̄, n, and n − 1.
How to Calculate Percentage Increase
Learn how to calculate percentage increase, use the percentage increase formula, and check your answer with a simple calculator.
Percentage Increase Formula
Learn the percentage increase formula, what each part means, and how to use it with simple examples for prices, pay, traffic, and other increases.
Quadratic Formula
Learn the quadratic formula, what each part means, how the discriminant works, and how to solve quadratic equations step by step.
Distance Formula
Learn the distance formula, how it works with coordinates, and how to find the distance between two points step by step.
Midpoint Formula
Learn the midpoint formula, what each coordinate means, and how to find the midpoint between two points step by step.
Slope Formula
Learn the slope formula, what each part means, and how to find the slope of a line between two points step by step.
Percent Error Formula
Learn the percent error formula, why absolute value is used, and how to calculate percent error step by step with chemistry and physics examples.
Z Score Formula
Learn the z score formula, how to calculate a z score step by step, and what positive, negative, and zero z scores mean in statistics.
How to Calculate a Loan Payment
Learn how to calculate a monthly loan payment using loan amount, APR, and loan term. See the loan payment formula, worked example, and extra payment impact.
How to Calculate a Mortgage Payment
Learn how to calculate a mortgage payment with principal, interest, taxes, insurance, PMI, and HOA dues. See the formula and worked example.
How to Calculate a Car Payment
Learn how to calculate a monthly car payment with vehicle price, down payment, trade in value, sales tax, APR, loan term, and fees.
Interest Only Loan Formula
Learn the interest only loan formula, how to calculate interest only payments, and why the principal balance does not go down during the interest only period.
Interest Only vs Amortizing Loan
Compare interest only payments and amortizing loan payments, including how each payment works, what happens to the principal balance, and when to use each calculator.
APR vs Interest Rate
Learn the difference between APR and interest rate, why APR can be higher than the stated rate, and which number to use when comparing loans.
How to Calculate Loan Interest
Learn how to calculate loan interest using simple interest, interest only payments, and amortizing loan examples.
Simple Interest vs Compound Interest
Learn the difference between simple interest and compound interest, how each formula works, and when to use each calculator.
When Do AP Scores Come Out in 2026? (July 6 Release Date)
2026 AP scores release on Monday, July 6, 2026 starting at 8 AM Eastern Time. Full schedule, how to access your scores, and what to expect after the College Board exam.
College Admissions Calculators
Free score calculators for every major college admissions test. Predict your SAT (400-1600), ACT (1-36), PSAT/NMSQT with National Merit Selection Index, and AP exam scores. Used by students nationwide.
AP Score Calculators
Free AP score calculators for every major AP exam. Predict your 1-5 score for APUSH, AP Calc AB/BC, AP Bio, AP Chem, AP Lang, AP Lit, AP Gov, AP World, AP Stats, and AP Precalc using the official section weightings.
Chemistry Calculators
Free chemistry calculators for molar mass, molarity, stoichiometry, dilutions, pH, limiting reactants, and more — each shows the formula and step-by-step steps.
PDF Tools
Free online PDF tools to convert, rotate, crop, split, delete, reorder pages, and add page numbers to your PDF files. Processed 100% locally.
How to Calculate GPA
Complete guide to calculating your GPA on the 4.0 scale. Learn the formula, see worked examples for weighted and unweighted GPA, and convert letter grades to grade points.
What Is a Good GPA?
A complete guide to what counts as a good GPA in high school and college. See national averages, college admissions benchmarks, scholarship thresholds, and graduate school requirements.
How to Calculate Your Final Grade
Step-by-step guide to calculating your final class grade. Learn how to handle weighted assignments, figure out what you need on a final exam, and convert points to percentages.
How to Calculate BMI
Step-by-step guide to calculating Body Mass Index (BMI). Learn the metric and imperial formulas, see worked examples, and understand BMI categories from underweight to obesity.
How to Calculate a Percentage
Step-by-step guide to calculating percentages. Learn the three main percentage formulas, see worked examples for finding percent of a number, percent change, and tips and discounts.
Acceleration Formula
Learn the acceleration formula, what each variable means, how to use it with units like m/s², and step-by-step worked examples.
Circumference Formula
Learn the circumference formula, how to calculate the distance around a circle using radius or diameter, and check worked examples.
Arc Length Formula
Learn the arc length formula, how to calculate the length of a curve along a circle's edge in radians or degrees, and see worked examples.
Volume Formula
Learn the volume formulas for cubes, boxes, cylinders, spheres, and cones, with worked examples and practical measurement applications.
Velocity Formula
Learn the velocity formula, how to calculate speed and direction, what each variable means, and check worked kinematic examples.
Area Formula
Learn the area formulas for squares, rectangles, triangles, circles, trapezoids, and parallelograms, with step-by-step worked examples for each shape.
Empirical Formula
Learn how to calculate the empirical formula of a chemical compound from mass or percent composition with a step-by-step worked example.
Arithmetic Sequence Formula
Learn the arithmetic sequence formula to calculate the nth term (a_n) and sum of a sequence, with step-by-step worked examples.
Percent Change Formula
Learn the percent change formula, how to calculate percentage increase and decrease, and see step-by-step worked examples.
Kinetic Energy Formula
Learn the kinetic energy formula, what each variable means, how to use it with SI units like Joules, and review worked physics examples.
Work Formula
Learn the work formula (W = F × d), how to calculate work at an angle with cosine, SI units (Joules), and step-by-step physics worked examples.
Recursive Formula
Learn how recursive formulas work, how they define sequences using previous terms, the difference between recursive and explicit formulas, and step-by-step math examples.
Point-Slope Formula
Learn how to use the point-slope formula (y - y1 = m(x - x1)) to find the equation of a line, see worked examples, and check when to apply it.
Surface Area Formula
Learn how to calculate the surface area of cubes, rectangular prisms, cylinders, spheres, and cones, with worked examples for each shape.
Explicit Formula
Learn how to use explicit formulas in sequences, compare explicit vs recursive formulas, and solve nth-term math problems step-by-step.
Perimeter Formula
Learn how to calculate the perimeter of squares, rectangles, triangles, and circles (circumference), with step-by-step worked examples.
Combination Formula
Learn how to use the combination formula (nCr) in statistics, understand the difference between combinations and permutations, and check worked examples.
Change of Base Formula
Learn the change of base formula (log_b(x) = log(x)/log(b)) to evaluate logarithms with any base using common or natural logs, with worked examples.
Geometry Formulas
A complete reference list of geometry formulas for 2D and 3D shapes, including area, perimeter, volume, surface area, coordinate geometry, and right triangles.
What Is a Quadrilateral? Definition, Types, and Formulas
A complete guide to quadrilaterals: definition, classifications (square, rectangle, parallelogram, trapezoid, rhombus), properties, and area/perimeter formulas.
What Is a Prime Number? Definition, Examples, and the Sieve of Eratosthenes
Learn what prime numbers are, the difference between prime and composite numbers, why 1 is neither, and how to find primes using the Sieve of Eratosthenes.
How to Find the Area of a Triangle: Formulas, Examples, and Math Rules
Master the triangle area formula (Area = ½ × base × height). Learn how to identify bases and perpendicular heights, solve step-by-step examples, and use Heron's formula.
How to Calculate Square Feet: A Practical Guide for Homes and Rooms
Learn how to measure and calculate square footage for rooms, yards, and irregular spaces, with worked examples and material ordering tips.
What Is a Rational Number? Definition, Examples, and Properties
Understand rational numbers, how they compare to irrational numbers, how to write them as ratios, and the difference between terminating and repeating decimals.
What Is a Factor in Math? Definition, Finding Factor Pairs, and Trees
Learn what factors are, the difference between factors and multiples, how to list factor pairs, and how to find the Greatest Common Factor (GCF).
How to Find Range in Statistics: Formula, Steps, and Examples
Learn how to calculate the statistical range of a dataset, see step-by-step examples, and understand how range compares to standard deviation and IQR.
What Is Mode in Math? Definition, Calculation, and Examples
Learn how to find the mode of a dataset, understand unimodal, bimodal, and multimodal distributions, and compare mean, median, and mode.
What Is a Compound Sentence? Rules, Coordinating Conjunctions, and Semicolons
Learn the definition of a compound sentence, how to join independent clauses using FANBOYS or semicolons, and how to avoid common comma splices.
What Is 90 Days From Today? How to Calculate Date Offsets
Learn how to calculate exactly 90 days from today or any starting date, count across different months, and check common calendar deadline rules.
What Is Annual Income? Gross vs. Net Salary Explained
Learn what annual income means, the difference between gross and net income, common pre-tax and post-tax deductions, and how to convert hourly wages to salary.
How to Open HEIC Files on Windows (Free, No Software)
HEIC files won't open on Windows by default. Here's how to view and convert iPhone HEIC photos to JPG or PDF for free — right in your browser, no software to install.
Browse by topic
Each topic groups the guides with the calculators that run the formula for you.
Statistics guides
Standardize a value against a distribution, measure how spread out a data set is, and read population versus sample statistics correctly.
Guides
- Z Score Formula
- Standard Deviation Formula
- Variance vs Standard Deviation
- Sample vs Population Standard Deviation
- How to Calculate Standard Deviation in Excel
- Standard Deviation Symbol
Related calculators
Formula guides
Walkthroughs of the standard high-school and college math formulas, with worked examples for each.
Guides
Related calculators
Percentage and error guides
Compare a measurement to an accepted value, or compute the percent change between two values.
Guides
Related calculators
Health and nutrition guides
Plain-English explanations of TDEE and resting metabolism, plus how the numbers translate to a daily calorie target.
Guides
Related calculators
Money guides
Step by step walkthroughs of how monthly loan, mortgage, and car payments are calculated.
Guides
Related calculators
Other guides
Background on the topics behind the smaller corners of the site.
Guides
Related calculators
How Resources fits in
Each calculator already shows its formula on the same page as the result. Resources is for the longer explanations that don't fit in a single panel — what a method actually means, where it comes from, and how to read the output sensibly.