Hot water that runs out early can come from two different causes. The tank may be too small for the way the house is used now. Or the tank may be holding less usable heat because sediment has settled on the bottom. From the tap, the two look alike. They are not the same problem, and they do not get the same answer.
Start with the morning
Pick the hour when the hot water runs out and write down what was running at that time. Note each shower, each hot load of laundry, the dishwasher, and any bath that was filling. Then note how many of those started back to back. One shower with the dishwasher running in another room is a different test from three showers that start within the same half hour.
Next, note how the water quits. If it turns cold all at once, household demand has passed what the tank can supply. If it goes lukewarm and then cold over a longer stretch, the usable heat in the tank may be reduced, and that is where sediment comes in.
Listen to the tank
A rumbling or popping sound while the unit is heating is a common sign that sediment has settled. Note when the sound happens. Does it occur only while heating, or whenever the tank is full? Does it get worse after a long run of hot water? Write these notes on the same page as the morning log so the pattern is visible.
Check the nameplate and the install year
Find the nameplate on the tank. It lists the capacity and the recovery rate for that model. Compare the capacity with the draws you logged. Then find the install year. If it is missing, rebuild it from the paperwork, a receipt, a permit card, or the year stamped on the label. Once you have it, the install year tells you how much weight the sound and the timing deserve. Age is a planning input, not a verdict.
Questions to ask before you choose a fix
Ask whether the tank size matches the household you have now, not the household that lived there at installation. Ask what the recovery rate means for back-to-back draws. Ask whether sediment is likely, what a flush would involve, and what a flush would not fix. Ask whether a larger tank, a second unit, or a change in how the house draws water is the better route for your log. If the answer changes what you buy, get it in writing.
What the log tells you
If several draws stack up and the tank is the right size for a smaller household, the household has outgrown the tank. If one ordinary draw runs short and the tank rumbles while heating, sediment is the first thing to check. If both show up, you can have both. Plan for each finding on its own terms, and bring the log with you to whoever comes to look at the unit.