Direct answer
Each bit's place value is a power of 2: rightmost is 1, then 2, 4, 8, 16... Sum the place values where the bit is 1.
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Binary input
Decimal output
About Binary → Decimal
Going from binary to decimal is how you read a register dump or a bit field as a normal number. 11111111 = 128+64+32+16+8+4+2+1 = 255.
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Frequently asked questions
Is there a quick way to do this in my head?
Memorize powers of 2 up to 1024. For longer values, group bits in fours, convert each to hex, then convert hex to decimal — usually faster than direct binary-to-decimal.