Unit Swap

5,400 seconds is 90 minutes, 128,000 bytes is 125 kibibytes, and both of them go wrong in exactly the same two ways: the factor used the wrong way round, and the wrong factor used confidently. Unit Swap deals eight conversions in a round — two each from metric length, metric mass, the clock and binary data sizes — with four answers to choose from. Every wrong answer on the board is the real result of one named mistake, so picking one tells you which factor you just used instead of telling you nothing. Every step on every ladder here is a whole number, so every answer is a whole number: nothing is rounded, and no answer is nearly right.

It runs entirely on your device: 0 network requests, works offline. No ads, no account, nothing to buy, and no betting, money or prizes of any kind.

1. Focus the board — click it, or press Tab until it is focused.
2. Read the top line. It gives you a quantity in one unit and the unit it has to become, and it tells you which of the two is the smaller one.
3. Decide the direction before the arithmetic. Going to a smaller unit you multiply, because it takes more of them to say the same amount. Going to a bigger one you divide. Getting this backwards is not a small error — it puts you out by the factor twice over.
4. Read the factor off the ladder underneath the board. It is always on screen, with the exact step between every pair of units in that family, and the rung the question is asking about is marked.
5. Move between the four answers with the up and down arrows. Left and right do the same thing, and the list wraps at both ends.
6. Press Enter to commit the answer under the cursor. A right answer moves you on; a wrong one rules that option out with a ✕, strikes it through, and spends one of your allowance.
7. Read the line underneath. After every answer it names the factor involved — which one that option used, where that number comes from, and what the real sum is.
8. Watch the dots. The wrong answers you are allowed show as ●●● in the status row and go out one at a time. When the last one goes out the round ends. Picking an option that is already ruled out costs nothing.

How do you know whether to multiply or divide when converting units?

Ask which of the two units is smaller, and ignore everything else for a moment. If you are moving to the smaller unit, the number gets bigger, so you multiply — 5 kilometres is 5,000 metres because a metre is small and it takes a lot of them. If you are moving to the bigger unit, the number gets smaller, so you divide — 5,400 seconds is 90 minutes because a minute is big and there are fewer of them. That one question settles the direction every time, without any rule to memorise, and it is worth settling before you touch the factor. Getting the direction wrong is the expensive mistake: multiplying by 60 where you should have divided does not put you a bit out, it puts you out by 3,600 — the factor squared. That is why the answer that is wildly too large or too small is usually the direction, not the arithmetic.

Why is a kibibyte 1,024 bytes and not 1,000?

Because computers count in twos and the prefix was borrowed from a system that counts in tens. Two to the tenth power is 1,024, which is near enough to a thousand that early engineers called it a kilobyte, and the two meanings have been tangled ever since. The IEC names — kibibyte (KiB), mebibyte (MiB), gibibyte (GiB) — exist to end the argument: those are always powers of 1,024, while kilobyte, megabyte and gigabyte in their strict sense are powers of 1,000. This is why a drive sold as 500 GB shows up as about 465 GiB: nothing was lost, the two labels just count differently. Unit Swap uses the IEC names throughout, and 1,000 is one of the wrong answers on every binary question precisely because it is the one people reach for.

Are the wrong answers just random near numbers?

No, and that is the point of the game. Each one is the exact result of a single named mistake applied to the same quantity: the factor used the wrong way round, the look-alike factor used the right way round (1,000 where the answer needs 1,024, 100 where the answer needs 60), the answer one decimal place too big or too small, the quantity carried straight across unconverted, or the factor applied twice. Pick one and the game prints which factor it used and where that number comes from. There is never more than one right answer: the quantity and the pair of units are chosen first and the answer is computed from them exactly, then the whole question space is checked before any of it can be dealt — every option on every board has to be a distinct whole number, and a question that cannot produce three distinct wrong ones is never dealt at all.

Why does one round mix length, mass, time and data?

Because the skill is choosing the factor and the direction, and that only stays visible when the ladder changes underneath you. Eight metric questions in a row stop being conversions and turn into "add three zeros" — you get quick at moving a decimal point and learn nothing about deciding anything. Switching from metres to minutes to bytes forces the decision back to the front every question. The round is dealt two from each of the four families and then shuffled together, so the mix is guaranteed rather than left to chance.

Is there any money, betting or in-app purchase in this?

None, and there never will be. No currency, nothing to buy, no extra tries to unlock for a fee, no prizes.

Does it work offline?

Yes. Once the page has loaded once, the whole game — the ladders, the factors, the wrong-answer rules and the explanations — is on your device. No server is involved in play at any point.

Can I play it with a keyboard or a screen reader?

Both. Arrows move between the four answers and Enter commits one. Every answer is a real button that announces its position and reads its number with the unit spelt out in full, so "125 KiB" is announced as 125 kibibytes rather than as three letters. The question is announced as a sentence, an answer that has been ruled out announces itself as ruled out and carries a ✕ as well as a strike-through, and the wrong answers you have left are counted dots rather than a colour — so nothing on the board depends on telling two shades apart.

Only neighbouring units are paired, so one factor is always the whole sum: kilometres to metres, hours to minutes, kibibytes to bytes. Kilometres straight to millimetres is not asked, because a question with two rungs in it tests stamina rather than understanding. Every ladder used here has whole-number steps, which is exactly why imperial and mixed conversions are absent — inches to centimetres is 2.54, and a game that promised exact whole answers could not ask it honestly. Metric length runs mm to km, mass mg to tonnes, time seconds to weeks, and data bytes to gibibytes, with quantities up to a million and answers up to a million. Months and years are left out: they are not a fixed number of days, so there is no exact factor to learn. The wrong-answer allowance is three or four, worked out from how many distinct traps the round's questions actually carry rather than from a fixed number.

Roman Read is the closest in shape — a short round of exact conversions where every wrong answer is a named rule error. Prime Sweep and Number Span are the other number-family games here, one turning on divisibility and the other on holding a sequence. Quick Count is the one to try next if you like answering under a budget.

Bookmark this page (Ctrl+D, or ⌘D on Mac) or install the app — it works offline the next time you need it.