Or, if you just need a super-fast approximation for a PIC 16C7x or similar
(in assembly)

Byte/2 - Byte/8 with all remainders dropped:

        bcf     STATUS,C
        rrf     BYTE,w  ;128=100%
        movwf   PERCENT

        swapf   BYTE,W
        movwf   TEMP

        bcf     STATUS,C
        rlf     TEMP,W
        andlw   0x1F
        subwf   PERCENT,F

Example: Byte = 0x80 (128)
divide by 2: 0x40 (64)
swap into TEMP, rotate left, and erase high 3 bits. TEMP=0x0E (14)
Subtract this value from percent: 0x32 (50%)

Not all values work out this nicely, but it will be close


----------
> From: Keith Howell <keithh@ARCAM.CO.UK>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: Conversion of byte to percentage
> Date: Thursday, May 21, 1998 8:53 AM
>
> adrian aslett wrote:
> >
> > I have a need to convert a byte value into a percentage.
>
> By coincidence, I have to do the exact same thing
> to show a quantity (0-255) on a display of 0-99 bars.
>
> unsigned char   cByte;
> unsigned int    nPercent;
>
>         nPercent = (((int)cByte)*100)/256;
>
> and the division by 256 is identical to >>8,
> or just take the high byte.
>         0--> 0,
>         255 -> 99.