Guides
Background knowledge for everyday calculations, explained with worked examples, so you can use the calculators with confidence.
How to Count Days Between Dates (Inclusive vs. Exclusive)
How to count days between two dates: inclusive vs. exclusive counting, days after vs. day N, business days, leap years, month-end dates and deadlines.
Percent vs. Percentage Points: What's the Difference?
Percent vs. percentage points with worked examples: how to calculate percentages and percent change, why stacked discounts don't add up, common mistakes.
Tsubo, Tatami and Square Meters: Room Sizes in Japan
How big is a 6-jo room in Japan? Learn how tsubo, tatami (jo) and square meters convert, why mat sizes vary by region, and how listings define one jo.
Celsius, Fahrenheit and Kelvin: Conversion and Mental Math
Exact Celsius, Fahrenheit and Kelvin formulas, why -40 matches on both scales, and how far the double-and-add-30 shortcut drifts for weather, fever, ovens.
How to Calculate the Electricity Cost of Appliances
Estimate any appliance's running cost: convert watts to kWh, apply your rate, include standby power and variable AC draw, and work out LED bulb savings.
T-Scores and Japanese Hensachi: How Deviation Scores Work
How deviation scores (T-scores, or hensachi in Japan) work: the formula, a worked example with mean and standard deviation, percentiles and limits.
Japanese Era Years Explained: Reiwa, Heisei and Showa to AD
How to convert Japanese era years (Reiwa, Heisei, Showa) to Western years and back, with era start dates, transition years and tips for Japanese forms.