What is training volume?

In one line

Training volume is the amount of exercise you did over a set period, usually counted as the number of sets or the total weight lifted.

What is training volume?
The 'volume' in 'I upped my volume this week'
You often see the word 'volume' in gym talk and workout logging apps. Volume simply means 'amount'. This post explains how the amount of training is counted and why there are two ways to count it.
In short
How much training you did
Training volume is how much exercise you did over a set period. It is different from how heavy the work was (intensity). Even with the same weight, doing more reps or more sets makes the volume bigger.
First, reps and sets
The basic units of volume
Lifting a weight up and lowering it once is called a 'repetition' (rep). A group of reps done in a row without resting is called a 'set'. For example, if you lift 10 times and then rest, you have done 1 set of 10 reps.
Method one: total weight
Weight x reps x sets
One method adds up all the weight you lifted. Doing 3 sets of 10 reps with 60 kg gives 60x10x3, or 1,800 kg. This number is called 'volume load'.
Method two: number of sets
How many sets per muscle each week?
The other method counts how many sets you did for one muscle group in a week. If you do 5 sets for your chest on two days a week, your chest volume is 10 sets. Muscle research and exercise guidelines mostly use this method.
Which sets count?
Sets taken close to failure
When counting sets, you count sets taken to failure or close to it. 'Failure' is the moment you cannot lift even one more rep. Sets far from failure, such as light warm-up sets, do not fit this method.
Counted by the week
The weekly total matters, not how you split it
Volume is usually counted per week. With the same volume, muscle growth did not differ whether it was done in one day a week or spread across many days. That is why people look at the weekly total rather than a single day.
Volume and muscle
More grew more, but each extra set added less
Doing 10 or more sets per muscle group a week grew muscle more than doing fewer. But each extra set added less and less. Doubling your sets does not double your muscle growth.
A common misconception
Is a bigger kg total always a better workout?
For muscle, no. Doing a light weight for very many reps easily makes the kg total big. But sets taken to failure grew muscle by a similar amount whether the weight was heavy or light. So for muscle, people count sets taken close to failure rather than the kg total.
Summary
Volume is the amount of training, often counted in sets
Training volume is how much training you did, counted as total weight lifted or as sets per week. For muscle, a bigger kg total does not make a better workout. Related terms: RPE and RIR, progressive overload, 1-repetition maximum.