Weight fluctuations are common and occur for a variety of reasons. It is possible to weigh more the day after you strength train, though it is typically because of temporary changes. It is unlikely that overnight you can "gain" four pounds of either muscle or fat.
It is more likely that the weight gain after lifting is due to a cause other than your strength routine the day prior.
If you weigh more the day after strength training, it's not because you've built a ton of muscle. It's most likely due to water-weight gain.
True Weight Gain
True weight gain occurs when you eat more calories than you burn. One pound of fat is equal to about 3,500 calories, notes Harvard Health. So unless you sleep walk and go on an overnight binge, it is unlikely that you can consume enough calories in one day to truly gain weight. If your weight has fluctuated, it is more often than not just a temporary response to something your body experienced.
Weight Gain After Strength Training
Strength training causes mini tears in your muscles, which is how muscle growth occurs according to the American Council on Exercise. These tears, however, can lead to fluid retention in the muscle and cause a weight gain immediately after your workout. This may last for a few days and could explain why you weigh more after you lift weights.
It used to be bandied about that muscle weighs more than fat, but a pound is a pound no matter what it's made of. The truth is that muscle doesn't take up as much room as fat. But they both still occupy space while one is growing and the other is whittling away; and not usually at the same rate. Your body can temporarily respond to weight training as it balances the muscle gain with the body fat reduction.
Read more: Do You Gain Weight When Lifting Weights?
People typically drink more water while they exercise and after, which leads to temporary water retention after weight lifting. On the contrary, if you didn't drink enough water while you lifted weights and you are dehydrated, your body may be retaining fluid. Additionally, hormonal changes in the body, especially in women, can cause daily water fluctuations, which doesn't reflect fat loss or muscle gain.
The U.S. Anti-Doping Agency suggests weighing yourself before and after your workout. In fact, it might even be advisable to weigh yourself before you gulp down your pre-strength-training routine liquids and after you've consumed liquid after training. Do this as an experiment and you'll get a better idea of where you stand.
Why the Scale Jumps
Weight gain can occur from eating or drinking more than you think you are. Especially after working out hard, you may sometimes think you "deserve" a bigger portion. The American Council on Exercise recommends "meal timing." Your glycogen stores have been depleted by your workout and you should eat within 30 to 60 minutes of working out.
Sodium-filled foods can also cause a jump in the scale. For the best indication on where your weight stands, weigh yourself only once a week instead of every day. Do it consistently in the morning, preferably without clothing, and monitor changes in your measurements as well. True weight gain will occur by gaining muscle or fat and consuming enough calories to support it.