Can I convert MAS to VIFT?

Not with a fixed rule. The gap is usually 2 to 4 km/h, but it depends on the athlete's anaerobic speed reserve, recovery and agility. Two players with the same MAS can have different VIFTs, which is the reason the test exists.

Which one should I use to set 15-15 intervals?

VIFT. Short intervals at 95 to 105 % of VIFT load players of different profiles about equally. The same percentages of MAS do not, because MAS says nothing about how much speed a player has above it.

Which test gives MAS?

Continuous or progressive tests such as the beep test, the VAMEVAL or the Cooper run. Their final speed is read as MAS, with a margin of error that depends on the test. The 30-15 IFT gives VIFT, not MAS.

Does a higher VIFT mean a higher VO₂max?

Not necessarily. VIFT combines several qualities, and a player can raise it with better repeated-sprint recovery or agility without any change in VO₂max. The test's author publishes an equation that estimates VO₂max from VIFT, age, sex and weight, with the usual error of an estimate.

VIFT vs MAS: what's the difference?

Two speeds, two questions

MAS answers: how fast can you run before your oxygen uptake stops rising? It is defined as the slowest speed at which you reach VO₂max. It is a property of your aerobic engine.

VIFT answers: how fast can you keep going through repeated 30 s efforts with 15 s of walking, with turns at both ends of a 40 m course? It is a result in a test, and it depends on more than your aerobic engine.

Why VIFT is higher

In the 30-15 IFT you run 30 seconds, then walk 15. Those short walks let you run at a speed you could not hold continuously. The source puts it simply: VIFT is not a direct reflection of MAS, and the difference comes from anaerobic velocity reserve, the speed you can add above MAS for short efforts.

The example the source gives is an athlete who reaches VO₂max at 17 km/h and ends the 30-15 IFT at 18.5 km/h. Across athletes the gap is usually 2 to 4 km/h. It is not constant.

What goes into each number

	MAS	VIFT
Aerobic power	Yes	Yes
Anaerobic speed reserve	No	Yes
Recovery between efforts	No	Yes
Cost of changing direction	No	Yes
Typical test	Beep test, VAMEVAL, Cooper	30-15 IFT
Test format	Continuous or progressive	30 s run, 15 s walk, 40 m shuttle

An example with two players

Two midfielders both have a MAS of 16 km/h from the beep test.

Player A has a large speed reserve and recovers fast. Their VIFT is 19.5 km/h.
Player B is a steady runner with less top speed. Their VIFT is 17.5 km/h.

Set a squad session of 15-15 intervals at 100 % of MAS and both run 16 km/h. Player A is working well below their capacity and Player B is close to theirs. Set the same session at 100 % of VIFT and they run 19.5 and 17.5 km/h: the load on each is similar, and the distances covered differ.

Which one to train with

Continuous running and long intervals: MAS is the reference. Continuous runs are set as a percentage of a MAS taken from an aerobic test.
Short intervals (15-15, 10-10) and repeated-effort work in team sports: VIFT. Buchheit’s prescriptions for them run at 95 to 105 % of VIFT.
Recovery and base runs: either, as long as you do not mix the two scales.

Never apply a percentage taken from one scale to a speed from the other. 90 % of a VIFT of 18 km/h is 16.2 km/h. 90 % of a MAS of 16 km/h is 14.4 km/h. They are different sessions.

Getting each number

MAS: run a beep test or a VAMEVAL. The calculators convert your level to MAS.
VIFT: run the 30-15 IFT. The 30-15 calculator converts your stage to VIFT and the training speeds for each zone.

If you only have a MAS and need to prescribe short intervals, test the squad on the 30-15 IFT rather than guess a VIFT from MAS. The gap is too variable to estimate.

What not to do

Do not read a VIFT as a MAS to set a continuous run: you would run too fast to finish.
Do not subtract a flat 3 km/h to get MAS from VIFT. It works on average and fails on the individuals you care about.
Do not compare a VIFT with a beep test level. They are different scales from different tests.