Every 10 bits multiplies by ~1,000 — kibi and kilo differ by just 2.4%.
2¹⁰ = 1,024. So 2²⁰ ≈ a million, 2³⁰ ≈ a billion — powers of two estimated in your head to within a few percent.
The 2.4% error compounds: by the fourth power it is 10%, so a 'terabyte' drive of 10¹² bytes shows as 931 GiB in an operating system counting in 2⁴⁰. That gap is not the manufacturer cheating, and it grows every time storage gets bigger.
At 10 the rule says 1,000 and the exact answer is 1,024 — 2.3% low. It holds to within 5% up to 21.6, then drifts.
The 2.4% gap at 2^10 is the same 2.4% every ten powers, so it compounds: it is exactly why a "1 TB" drive shows up as 931 GB.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Bit Calculator →Every 10 bits multiplies by ~1,000 — kibi and kilo differ by just 2.4%. 2¹⁰ = 1,024.