1Roman numerals stop at 3,999
3,999 is MMMCMXCIX. There is no standard symbol for 5,000 in the notation everyone learned, so 4,000 cannot be written at all without inventing a bar or a bracket.
Decimal to Roman numeralsThings people get wrong
Every figure below recomputed · How we check
Twelve claims that sound wrong and are not. Each one is computed by the same evaluator that runs the calculators, and each links to the page that shows the arithmetic — so none of these has to be taken on trust, including from us.
21 January 2027 falls in week 53 of 2026
ISO 8601 gives a week to the year that owns its Thursday, and that week’s Thursday is 31 December 2026. 2026 has 53 weeks; 2027 has 52.
Week numbers 20263A cup of flour varies by a third
Spooned it is 120 g, dipped 148.8 g, packed 159.6 g. A cup of sugar weighs 198 g however you fill it, because sugar does not compress and flour does.
Grams to cups4The inch is exactly 25.4 mm, and only since 1959
Before the 1959 international yard and pound agreement the American and British inches differed by about 2 parts per million. The yard was then defined as exactly 0.9144 m, which fixes the inch at 25.4 mm with no error bar and no further decimals.
Inches to centimetres5An acre is exactly 43,560 square feet
It is one chain by one furlong — 66 feet by 660 — which is why the number looks arbitrary and is not. It was the area a yoke of oxen could plough in a day.
Acres to square feet6Black Friday is defined by Thanksgiving, not by a date
Thanksgiving is the fourth Thursday of November — 26 November in 2026 — so Black Friday is the 27th. Neither has a fixed date, and both move by up to a week between years.
November 20267Easter can move by 35 days
It is the first Sunday after the first ecclesiastical full moon on or after 21 March, which puts it anywhere from 22 March to 25 April. In 2026 it is 5 April; in 2027, 28 March.
Easter moves, but not randomly82026 has 261 working days
365 days, 104 of them weekend days, leaves 261. Public holidays are not deducted, because which ones apply depends on where you are and a single national figure would be wrong for most readers.
2026 calendar9A litre of water weighs almost exactly a kilogram
Not a coincidence: in 1795 the kilogram was defined as the mass of a litre of water. Both have since been redefined independently, which is why the agreement is now to about 1 part in 30,000 rather than exact.
Litres to gallons10A US fluid ounce and an imperial fluid ounce are different
The US one is 29.5735295625 ml, the imperial 28.4130625 ml. A UK recipe asking for 10 fl oz means 284 ml; an American one means 296 ml. Both are exact figures, and they disagree by 4%.
Fluid ounces to millilitres11An ounce of flour is not a fluid ounce of flour
One is a weight and the other a volume, and the word is the same. A fluid ounce of all-purpose flour is about 15 g; an ounce of it is 28.35 g — near enough a factor of two, in the direction that ruins the dough.
Grams to cups12Binary 1011 is 11, and you can check it in four multiplications
1×2³ + 0×2² + 1×2¹ + 1×2⁰ = 8 + 0 + 2 + 1. Every base conversion is that sum; nothing is rounded, so the result can be verified digit by digit.
Binary to decimal