Wine leaves a barrel whether you sell it or not. Between three and six percent a year evaporates through the oak, and every gallon of it has to be replaced from somewhere or the ullage turns into oxidation. The topping reserve is not a rounding error — on a hundred-barrel program it is a couple of hundred gallons you have to plan for at press, not discover in March.
Sixty 59-gallon barrels losing 4% a year need 141.6 gallons of topping wine — about 11.8 gallons every month.
The example
Three lots in the same cellar, each at a different loss rate depending on barrel age and cellar humidity.
| A | B | C | D | E | F | G | |
|---|---|---|---|---|---|---|---|
| 1 | Lot | Barrels | Gal/barrel | Annual evap | Gal lost/yr | Gal per topping | Cases lost |
| 2 | Cabernet 2025 | 60 | 59 | 4.0% | 141.60 | 11.80 | 59.6 |
| 3 | Syrah 2025 | 24 | 59 | 5.5% | 77.88 | 6.49 | 32.8 |
| 4 | Chardonnay 2025 | 100 | 59 | 3.5% | 206.50 | 17.21 | 86.9 |
The formula
One multiplication, then two ways of looking at it:
How it works
Three inputs and two useful outputs:
B2*C2is the volume under oak. A standard barrique is 225 litres, about 59.4 gallons; use your cooper's actual figure because puncheons and hogsheads change it substantially.*D2applies the loss rate. Three to four percent is typical for a humid cellar; five to six for a dry one or for new oak, which is thirstier in its first year./12turns the annual figure into a topping schedule. Most cellars top monthly, and the reserve has to be available at each of those visits, not just in total./2.3775converts gallons into 12-bottle cases — nine litres each. That is the number to put in front of anyone who thinks evaporation is not a real cost.
Hold the topping reserve back from the same lot, in a topped and inert-gassed vessel. Topping Cabernet with something else is a blending decision, and it should be a deliberate one.
Try it: interactive demo
Enter your barrel count, barrel size and the annual evaporation rate.
Variations
Reserve barrels to set aside
Divide the annual loss by barrel volume to see how many full barrels the topping programme consumes. It is usually more than a cellar expects.
Value of the loss
Multiply cases lost by your wholesale case price. This is the line that justifies a humidifier.
Pitfalls & errors
Evaporation is not linear across the year. A new barrel loses most heavily in its first months and a dry summer spikes it — if you top on a schedule, expect the early toppings to take more than the average.
Cellar humidity is the lever you actually control. Raising relative humidity shifts the loss from water to alcohol, which reduces volume loss but raises alcohol concentration; lowering it does the reverse. Neither is free.
Do not size the reserve from the annual total alone. If the wine is in barrel for eighteen or twenty-four months, multiply by the full time under oak — a two-year Cabernet needs roughly twice this number.
Practice workbook
Frequently asked questions
Is the angel's share the same as topping loss?
Why not just leave the headspace?
Stop fighting formulas. Learn them in a day.
This recipe is one of hundreds of real-world formulas we teach. Our Excel Formulas & Functions class covers lookups, logic, text, and dynamic arrays hands-on — live in Dallas–Fort Worth, Houston, Austin, Oklahoma City, Denver, or online.
See the Formulas & Functions Class