- What is OG and FG, and why do I need them?
- OG (Original Gravity) is the specific gravity of your wash before fermentation — a measure of dissolved sugars. FG (Final Gravity) is the reading after fermentation completes. The difference tells you how much sugar was converted to alcohol. A typical sugar wash might start at OG 1.080 and finish at FG 1.005, giving an ABV of about 9.8 %. This wash ABV is important because it sets an upper bound on what you can recover from the still and lets you calculate yield efficiency.
- How accurate is the (OG − FG) × 131.25 formula?
- It is accurate to within ±0.3–0.5 % ABV for standard fermentations with OG below 1.100. The formula assumes a linear relationship between gravity drop and ethanol production, which holds well in the typical homebrewing range. For high-gravity washes (OG above 1.120) or washes containing non-fermentable sugars, use the more precise alternate formula: ABV = (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794). Both give essentially the same result for OG under 1.100.
- What is Pearson's Square and why use it for dilution?
- Pearson's Square is a simple mass-balance technique: the amount of pure alcohol before dilution must equal the amount after. Expressed algebraically: V₁ × C₁ = V₂ × C₂, where V is volume and C is ABV. Rearranging gives V_water = V_spirit × (C_spirit / C_target − 1). For example, diluting 2 L of 65 % spirit to 40 %: water needed = 2 × (65/40 − 1) = 2 × 0.625 = 1.25 L, producing 3.25 L at 40 %. The formula ignores the small volume contraction that happens when ethanol and water mix (~2 %), which is acceptable for home production but worth correcting if bottling for sale.
- My yield efficiency is over 100 % — is that an error?
- It usually means one of two things: (1) your FG reading was taken before fermentation fully completed, so the estimated wash ABV is too low; or (2) you are measuring the spirit ABV from your parrot or hydrometer at a temperature other than calibration temperature (usually 20 °C), and the reading is off. Re-take your FG when the gravity has stabilised for 48 hours, and temperature-correct your spirit hydrometer reading. True distillation efficiency cannot exceed ~100 % — a pot still cannot create alcohol that was not in the wash.
- Can I use this to calculate how much to dilute new-make spirit before ageing?
- Yes. Many distillers enter barrels at 55–63 % ABV (110–126 US Proof) to allow good wood extraction while controlling alcohol loss during ageing. Enter your new-make spirit ABV and volume, then set the target ABV to 63 % (or your preferred entry proof) to find exactly how much water to add before barrelling. Remember to use filtered, soft water (ideally reverse-osmosis or low-mineral spring water) to avoid calcium and chlorine affecting the final flavour.