Formula: V_out = V_in × R2/(R1+R2). If a load R_L is connected across the output, it sits in parallel with R2 — use R2' = (R2×R_L)/(R2+R_L) in place of R2, which always lowers V_out below the unloaded calculation ("loading effect"). This is why voltage dividers make poor voltage sources for anything but very light, high-impedance loads (like an op-amp or ADC input) — for real loads, use a proper voltage regulator instead.