*Roasting machines vary hugely in size, from industrial machines such as the Probat PX120 (left), with a capacity of 120 kilograms, to tabletop roasters such as this 1-kilogram model (right), designed for use in cupping labs.*

When deciding what size of roasting machine to purchase, the first thing to work out is how much coffee you need to roast per week. For a comfortable roasting program, we recommend that you purchase a machine that can roast this amount of coffee in no more than 25 hours per week.

Almost invariably, a roasting machine’s real capacity will be less than its stated capacity, thanks to a number of factors:

#### Weight Loss

Typical weight losses for roasted coffee range from 14–20%, the lesser amount for light roasts and closer to 20% or more for very dark roasts. The weight loss for beans from certain prominent coffee chains can be as much as 22–25%.

#### The Burner Capacity

A machine’s burner capacity is the real limiting factor on how much coffee a machine can roast per hour.

*Roasting 1 kg of green coffee in a reasonable time requires a burner capacity of approximately 11,500 kJ/hour.*

(Alternatively, roasting 1 lb. of coffee requires about 5,000 BTU/hour.) A machine that recirculates hot air back into the roasting chamber, such as a Loring, will have a slightly higher roast capacity.

#### Overfilling

Whilst a roaster stuffed full of beans will accommodate well beyond its stated capacity, this also means the coffee will take longer to roast. A salesperson may claim that you can roast 15 kilograms per batch in a 15-kg machine. But to make sure your drum roaster is never underpowered, use the following rough formula to estimate your effective roasting capacity:

When planning your roasting program, assume you will roast 3–3.5 batches of green coffee per hour at 50–70% of a machine’s stated capacity. Then deduct the 14–20% weight lost per batch to calculate how much roasted coffee per hour a machine can produce.