Best, fast & exact floor(sqrt()) for Arduino etc.

sqrt.c

uint8_t my_sqrt(uint16_t input) {
    uint16_t res = 0;
    uint16_t one = 1u << 14;

    while (one > input) one /= 4;
    
    while (one != 0) {
        if (input >= res + one) {
            res += one;
            input -= res;
            res += one;
        }
        res /= 2;
        one /= 4;
    }
    
    return res;
}