How we test our calculators

In short

• Each formula comes from a primary source, which the calculator page names and links.
• Each calculator is checked against a set of inputs whose correct answers are known in advance, including awkward cases such as zero, negative numbers, blank boxes, very large numbers and leap years.
• The tests are run again on the code exactly as it is published, and then on the live site after each update.
• Every worked example on a calculator page is produced by the calculator itself.
• When we find a mistake, we fix it, add a test so it cannot come back, and record it in the corrections log below.

This process applies to every calculator we update. In October 2026 all 49 calculators listed in our sitemap were rebuilt or re-tested this way, and they have more than 900 such tests between them.

1. Start from the definition

A calculator is only as good as the rule it implements, so the first step is to find the rule where it is defined, not where someone else has repeated it. Some examples:

• The working days calculator follows the federal holiday rules in 5 U.S.C. 6103 and the U.S. Office of Personnel Management's schedules, including the rule that a Saturday holiday is observed on the Friday before.
• The SAT to ACT converter uses the official 2018 ACT/SAT concordance tables, not a table copied from another site.
• The weight converter uses the exact legal definition, 1 pound = 0.45359237 kilograms, as published by NIST.
• The CFA Level 1 calculator follows the sign convention in the Texas Instruments BA II Plus guidebook: money paid out is negative and money received is positive.

2. Known-answer tests

For each calculator we write down inputs together with the answer they must give, worked out by hand or taken from the source, and check the calculator against all of them. For example, the working days calculator must count 261 weekdays in 2026 and 250 business days once the 11 federal holidays are taken out, and 249 business days in 2027, because New Year's Day 2028 falls on a Saturday and is observed on Friday, December 31, 2027. The SAT to ACT converter is checked against rows of the official concordance.

The tests cover the cases that tend to break calculators:

• zero, negative numbers, decimals and blank boxes;
• very large and very small numbers;
• inputs that have no valid answer, such as the square root of a negative number or lengths that cannot form a triangle;
• dates entered in the wrong order, leap years and month ends (January 31 plus one month);
• rounding at the edge, such as a reading time that would otherwise show "2 min 60 sec".

The tests run on the code exactly as it is stored for the site, and again on the code the live site serves after each update. An update does not count as finished until every test passes on the live site.

3. No answer is better than a wrong answer

When the inputs have no valid result, a calculator should say so rather than show a number. The Pythagorean theorem calculator reports "Not a triangle" for sides of 1, 2 and 10. The CFA Level 1 calculator shows no interest rate when every cash flow has the same sign, because no rate can balance such a problem, and its results line explains why. The binary, decimal and hex converter rejects a digit that does not belong to the chosen base, such as 2 in binary, instead of quietly ignoring it.

4. Exact arithmetic where it matters

Computers store most decimals approximately, so 0.1 × 3 comes out as 0.30000000000000004. That is harmless in a displayed result, but it can tip a rounding decision or an equality test the wrong way. We handle it in three ways:

• Exact whole numbers. The GCF and LCM calculator, the binary converter and the perfect-root test in the square root calculator use exact integer arithmetic, so even very large numbers give exact answers.
• Exact forms alongside decimals. Where a school answer is expected in exact form, the calculator shows it: √72 = 6√2, a slope of −2/3 rather than only −0.666667, and the factors behind a GCF.
• Deliberate tolerances. Where a comparison must allow for tiny computer errors, the tolerance is stated on the page, such as the one-part-in-a-million test the Pythagorean calculator uses to decide whether a triangle is right.

Dates are handled as calendar dates in your own time zone, so a date never shifts by a day because of where you are. The date calculators are tested both on a computer set to India Standard Time and as a visitor in New York, where daylight saving time adds a further twist.

5. Every number on the page comes from the calculator

Worked examples are not typed in by hand. Each one is produced by running the calculator on the page with the inputs shown, and the text quotes its output exactly. Before the page goes live, every claim it makes about the tool, such as "rounds to four decimal places" or "gives no result", is checked against what the tool actually does. See the editorial policy for how sources and updates are handled.

6. The calculations run in your browser

The calculators run entirely in your browser. The numbers you type are not sent to us, and the same inputs always give the same result. The site uses Google Analytics to count page visits; the privacy policy has the details.

7. What testing cannot promise

• The last decimal place. Results with many decimal places come from floating-point arithmetic, and the final digit can occasionally be off by one.
• Estimates are estimates. Some calculators rest on averages or rules of thumb, such as average reading speeds or the number of words on a page. The page says when this is the case.
• Your own rules. Schools, employers and exam boards set their own policies, such as attendance requirements or the federal holidays your employer actually observes. Check the official rule that applies to you.
• Professional advice. The calculators are for education and everyday planning; see the disclaimer.

Corrections log

Errors found in calculators that were live on the site, with the date each fix went live. Each fix comes with a new test, so the same error cannot return unnoticed.

October 17, 2026

• Income tax calculator and paycheck calculator: the 2026 head-of-household 24% bracket ended at $201,775 instead of $201,750 (IRS Rev. Proc. 2025-32), which put a few dollars too little tax on head-of-household incomes above $201,750. The paycheck calculator also had no 401(k) limit: 20% of a $200,000 salary counted as $40,000 of contributions. It now stops at the 2026 limit of $24,500.
• BMI calculator and body fat calculator: the category could disagree with the number shown. A BMI of 24.98, displayed as 25.0, was labelled normal weight, and the body fat page's own example, 18.0%, was labelled Fitness instead of Average. Impossible inputs, such as a height of 20 cm, also gave a result. Categories now follow the number as displayed, BMI uses the CDC's category names, and impossible measurements give a message instead of a number.
• Calorie calculator: the minimum target for men was 1,200 calories a day instead of 1,500, and for small or older adults a "weight loss" target could be higher than their maintenance calories. It also accepted children, although the equation it uses was developed for adults. Loss targets now respect both minimums, and the calculator is for adults only.
• Credit card payoff calculator: payoffs longer than 100 years were cut off at 1,200 months without warning, so the totals were wrong; at 0% APR the interest could even show as negative. Every payment is now counted.
• Retirement calculator and savings goal calculator: the retirement calculator ignored the IRS contribution limits and treated a negative return differently for the balance and the contributions. The savings goal calculator showed negative interest when the starting balance already reached the goal.
• Percentage calculator, tip calculator and GPA calculator: a percentage change from a negative starting value had the wrong sign, and "7% of 300" read 21.000000000000004. The tip keypad rounded some amounts down by a cent ($38.05 instead of $38.06). The GPA calculator skipped grades typed with a typographic minus, such as A−.
• Mortgage and auto loan calculators: 0% loans showed "-0.00" of interest, and negative amounts were accepted. Both now give exactly 0 interest at 0% and reject negative inputs.

October 16, 2026

• Age calculator, date difference calculator and pregnancy due date calculator: dates were read as midnight UTC, which moved every date back a day for visitors in time zones west of UTC, including the whole United States. On someone's 26th birthday, for example, the age calculator showed 25 years, 11 months, 30 days, and due dates came out a day early. All three now read dates as calendar dates in your own time zone.
• Algebra equation solver: an error in the calculator's code stopped it from showing any result. It now solves linear equations, quadratics and systems of two equations, with steps.
• CFA Level 1 calculator: when no interest rate could solve a problem, it still showed one, for example 624.7626% for PV = 1,000 and FV = 2,000. Solving for the number of periods could also give a negative answer (−14.21). It now shows no result and explains why.
• Binary, decimal and hex converter: invalid digits were ignored without warning, so 102 entered as binary was read as 10 (2 in decimal), and numbers above about 9 quadrillion lost their last digits (12345678901234567890 came out as 12345678901234567000). Invalid digits are now rejected and large numbers are exact.
• Working days calculator: dates were read as midnight UTC, which moved every date back a day for visitors in time zones west of UTC, including the whole United States. Holidays now also follow two more OPM rules: when New Year's Day falls on a Saturday, it is observed on Friday, December 31 of the year before, and Juneteenth is counted only from 2021, when it became a federal holiday.
• Class attendance calculator: a floating-point rounding error could ask for one class too many. With a 28% requirement over 25 classes, for example, it asked for 8 classes instead of 7.
• Reading time calculator: times could read "2 min 60 sec", and two reading-speed presets (300 and 500 words per minute) had no source. The presets now use the averages from Brysbaert's 2019 review of reading-rate studies.

October 16, 2026 (earlier the same day)

• SAT to ACT converter: the conversion table did not match the official 2018 concordance. SAT 680 gave ACT 12 (official: 11) and ACT 36 gave SAT 1570 (official: 1590). The table was rebuilt from the official concordance, and unsourced percentile and "school tier" labels were removed.
• Date add and subtract calculator and days until calculator: the same UTC problem moved dates back a day for visitors west of UTC, and adding one month to January 31, 2026 gave March 3. It now gives February 28.
• Quadratic equation calculator: complex roots could be written with a negative imaginary part, such as x = 1 ± −2i for −x² + 2x − 5 = 0. They are now written as x = 1 ± 2i.
• Cumulative GPA calculator: a term GPA above 4.0 (up to 4.5) was shown as a reachable target even on a 4.0 scale. The calculator now asks for the grading scale and says when a target cannot be reached in one term.

October 15, 2026

• TI-84 calculator: several keys did not work or did not behave as they do on a real TI-84, including e, ANS, 2π and a negative number squared (−3²). They were fixed to match the TI-84.

Report a problem

If a calculator gives a result you think is wrong, please tell us the inputs you used and the answer you expected. We will check it, and a confirmed error will be fixed, tested and added to this log.

Last updated: October 17, 2026