Most modern laptops last 4 to 12 hours on a single charge. The exact figure for your laptop comes from one division: battery capacity in watt-hours divided by average power draw in watts.

The top-ranking guides give the same 4 to 12 hour range, but they leave three gaps. First, they rarely show the watt-hour division itself. Second, they seldom break runtime down by task, which is what actually drains a battery. Third, they rarely say where to find your battery's watt-hour rating. This guide closes all three.

The formula is runtime in hours = battery watt-hours / average watts. Find the watt-hour rating on the battery label, in your system battery report, or from the manufacturer's specifications. If only milliamp-hours and voltage are listed, convert first: watt-hours = milliamp-hours x volts / 1,000. For example, 7,500 mAh x 7.7 V / 1,000 = 57.75 Wh.

Worked example — light use. Take a 56 Wh battery with an average draw of 8 watts while browsing and writing documents. The runtime is 56 / 8 = 7 hours. The same battery streaming video at 12 watts gives 56 / 12 = 4.67 hours, which is 4 hours and 40 minutes. Gaming at 25 watts gives 56 / 25 = 2.24 hours, about 2 hours and 14 minutes. One battery, three very different answers, because the task sets the watts.

Worked example — an aged battery. Batteries lose capacity with use, and a battery at 80% health holds 56 x 0.80 = 44.8 Wh. At the same 8-watt light load, the runtime drops to 44.8 / 8 = 5.6 hours. If your laptop once lasted 7 hours and now lasts under 6 on the same work, capacity loss, not a setting, is the likely cause.

Typical runtimes by laptop type, for mixed real-world use. Ultrabooks and thin-and-light laptops usually manage 8 to 12 hours. Mainstream laptops manage about 5 to 8 hours. Gaming laptops and mobile workstations manage about 2 to 4 hours, and less while gaming. If your result from the formula sits far outside the range for your type, check for a runaway background app or a battery that needs replacing.

Three settings that genuinely move the number. Lowering screen brightness cuts one of the largest single power draws. Closing unused browser tabs and background apps lowers average watts. Switching to the battery-saver or balanced power mode caps processor speed, which matters most during heavy work. Each change lowers the watts in the division, which raises the hours.

Bottom line: ignore headline battery claims and divide watt-hours by watts for your own workload. A 56 Wh battery gives 7 hours of light work, 4 hours 40 minutes of video, and about 2 hours 14 minutes of gaming, and an aged battery gives less of each.

Q: How do I find my laptop battery's watt-hours?

A: Check the battery label, your system battery report, or the manufacturer's specifications. If only milliamp-hours and volts are listed, use watt-hours = milliamp-hours x volts / 1,000.

Q: Why does my laptop last far less than the advertised hours?

A: Advertised figures use light, standardized tests. Heavier tasks draw more watts, and the runtime formula divides the same watt-hours by a larger number, so the hours fall.

Q: How long should a gaming laptop battery last?

A: About 2 to 4 hours in mixed use, and less while gaming, because a dedicated graphics card and fast processor draw far more watts than a thin ultrabook.

Q: Does a battery at 80% health really lose that much runtime?

A: Yes. A 56 Wh battery at 80% health holds 44.8 Wh, so light-use runtime falls from 7 hours to 5.6 hours at the same 8-watt draw.