When lifting, do I count both dumbbells' weight or one..?

It only depends on how many arms are involved in the lift. Hopefully this is clear and will help:

1. If you're lifting 15 lb. dumbells, with one in each hand, then you are only lifting 15 lb. with each arm. Say you do 10 reps with each arm. Then you've done 10 reps x 2 arms x 15 lbs, which comes out to 300 lb for that set. When you log this, you should log 15 lbs. as the weight used.

2. If you're lifting a 30 lb. barbell, which requires both hands/arms, then you are balancing 30 lbs. between the two arms. Say you do 10 reps with the barbell. Then you've done 10 reps x 30 lbs, which comes out to 300 lb for that set. When you log this, you should log 30 lbs.

You've lifted the same amount of weight in a single set for both exercises, but dumbells should only be logged based on the weight of a single dumbell.

Say I'm doing shoulder presses with two fifteen pound dumbbells. Do I log my workout as lifting 15 lbs or 30? I've been doing former b/c I asked around on FB before I started looking at the community stuff on here, and a friend said to count the one weight, so in my example above, it would be 15. I've been logging my lifting in that manner for a few months now but today I was perusing the message boards and saw someone count both weights at once.. Which is right?

It doesn't particularly matter. If you can lift 15 pound dumbells in each hand, you can lift a 30 pound bar with both hands. Generally, people are stronger lifting a bar than they are lifting dumbells since it requires less stabilization. In other words, I can bench press more than I can dumbell press. So log it as 15 pound DBs and keep lifting!

Say I'm doing shoulder presses with two fifteen pound dumbbells. Do I log my workout as lifting 15 lbs or 30? I've been doing former b/c I asked around on FB before I started looking at the community stuff on here, and a friend said to count the one weight, so in my example above, it would be 15. I've been logging my lifting in that manner for a few months now but today I was perusing the message boards and saw someone count both weights at once.. Which is right?

I would notate:

DB Press 15# x10

to signify that the dumbbells were 15 lbs each.

If I was using a barbell (which I usually do) with say....135 on it I notate

Strict Press 135 x 5

As to which is right: Whichever one you feel comfortable with and allows you to remember what you did last time

Can I just ask a further question...?

How does dumbbell weight relate to machine weight for a similar movement? I'm thinking about shoulder presses in particular. If you're shoulder pressing 5kg with dumbbells, does that relate to 5kg or 10kg on the machine?

Realistically, I don't think it matters muich how you log it as long as you are tracking the weights (either way) and create goals for yourself to increase those weights. You will notice an increase in strength whether you track yourself going from 60-lbs to 70-lbs (total), or 30-lbs dumbbells to 35-lbs dumbbells (individually).

I think it depends on if you're lifting them at the same time or separately. If you have a 15lb weight in each hand and are curling or lifting both arms at the same time, then I'd count it as 30lbs, but if you do your right arm and then switch to your left arm, I'd count it as 15lbs. With MyFitnessPal though you could specify 15lbs per hand together or separately though so it gives more detail.

The weight will be the same regardless of if you use a dumbbell or a machine to perform your move. However, dumbbell moves generally incorporate more stabilization muscles (IE your abs) which offer a better workout.

Actually no, it won't be because machines have pulleys which take the burden of some of the weight. When I do a chest press with dumbbells the most I've done on my last set is 45 lbs. If I use the chest press machine the most I can do is close to 80 lbs.