Question on weight and VO2 max

Please note that VO2max calculation as done by Garmin devices is a very rough estimate, so I do not think that 1 kg difference matters at all (i.e. it might give different value, but neither will be accurate).

ok but if you were to try and very accurately measure vo2 max in some other way, what weight should you be using or does it not really matter?

The most accurate direct method of measurement (done in a lab) does not involve body weight at all.

Estimates are done using different methods, and some of them do not involve weight either. The firstbeat method which is probably used in garmin devices does utilise body weigth, but since the algorithm is proprietary I don't have any knowledge about the impact of body weight difference. But since the calculation is done during GPS-enabled activity, then I would use the weight you have during workout, so depending on the time of day of your workout I'd use a value closer to your morning or evening weight.

The units of VO2max are ml/min/kg. If you calculate VO2max in the lab the results are given in absolute units - L/min. This is then made into a relative value based on body mass, hence ml/min/kg.

But it's moot really because VO2max is not a measure that should be used to assess progress in the short term. It's a measure that you should use to track progress over a period of time - months not days. So whether or not you use your body mass measured in the morning or evening is largely irrelevant as the change is unlikely to matter.

to be more specific my weight has been dropping over the last 4 months, by about 9kg so I am not tracking short term VO2 max. I have been updating my weight in increments of 1kg on average and quite often as I update my weight, my VO2 max increases, although my general fitness has been improving as well with my training. I was just curious if I dropped my weight suddenly by another 1.5kg whether that would give me a more accurate VO2 max.

Maybe in the big scheme of things it doesn't really matter

if I dropped my weight suddenly by another 1.5kg whether that would give me a more accurate VO2 max

It would not necessarily be more accurate, but it would be different. It would show an increase.

Since your watch is only giving you an estimate and only in whole numbers there's only going to be so much precision to it. If you also take into consideration that weather will play a role then you really need to take the number it gives you with a huge grain of salt and just look at trends over the course of months.

As an example: Over the course of the winter last year my Vo2Max score slowly rose to 53. It stayed there through the spring when I backed off training for several weeks remained at 53 through the summer when I started training for my fall marathon. I could tell I was getting more fit but my Vo2Max score wasn't going up because it was summer in the Mid-Atlantic area and my HR was higher than usual for the same paces. Then we had about 2 weeks of autumnal weather and Vo2Max went up to 54 after the first week and a few days before it got hot and humid again it went up to 55. Three days later the heat was back and my Vo2Max score dropped back down to 53 and there it remains.

I didn't suddenly get more fit and then lose it again over a couple of weeks but the big swings in weather gave me far different estimations of Vo2Max due to it not being a direct measure of actual oxygen consumption.

Let,s assume your VO2max is 40 and your weight drops by 1 kg from 80kg to 79kg. That will change the calculated VO2max to 40,5 IMHO.
J.

Consider also that it's impossible for VO2max to continually rise. Big changes can be expected when coming from a low fitness base, but eventually those changes diminish to a point where you cannot expect to see changes, just maintenance.

to be more specific my weight has been dropping over the last 4 months, by about 9kg so I am not tracking short term VO2 max. I have been updating my weight in increments of 1kg on average and quite often as I update my weight, my VO2 max increases, although my general fitness has been improving as well with my training. I was just curious if I dropped my weight suddenly by another 1.5kg whether that would give me a more accurate VO2 max.

Maybe in the big scheme of things it doesn't really matter

If you can run a certan distance on a given distance.

If you run 5Km in 25 minutes with a weight at 80 Kg it gives you a Vo2Max (a number)
If you run 5Km in 25 minutes with a weight at 60 Kg it gives you a Vo2max. (a lower number). (Less weight should be able to run faster)