Ask someone how much storage their photos use and they will usually answer with a count: "about two thousand photos." That number feels informative, but it hides the two facts that actually decide storage: how large the average photo is, and how many videos sit beside the photos. A thousand small photos can use less space than ten long videos, and a library of high-resolution photos uses far more than the same count of compressed ones. Storage planning needs sizes, not just counts, and it needs the videos counted separately, because videos are typically fifty times larger than photos.
The calculation itself is straightforward. Multiply the number of photos by their average size, multiply the number of videos by their average size, add any other photo-related storage, and compare the total with the storage plan. The result answers three questions at once: how much you use now, how much headroom remains, and how many more photos would fit in that headroom. Those three answers are enough to decide whether to upgrade a plan, compress, archive, or simply carry on.
The iCloud Photos Storage Calculator tool performs that calculation from numbers you type in: photo count and average photo size, video count and average video size, other storage, and a plan size to compare against. It never connects to an iCloud account, never logs in, never syncs, and never fetches anything from Apple. It works only on values you provide, in your browser. Your library is never read, uploaded, or examined.
How the calculation works
The tool multiplies each count by its average size, then adds the results together with any other storage you enter. The total is compared with the plan size you entered, producing a percentage used, a remaining amount, and an estimate of how many more photos would fit in the remaining space. That last figure is derived by dividing the remaining space by the average photo size, which assumes future photos resemble past ones in size. If your future photos are larger, because you switched to a higher-resolution camera, the estimate will be optimistic, and you should adjust the average size upward.
Average sizes are the fragile input, because they vary enormously. A phone photo might average 3 to 5 megabytes, but a raw or uncompressed photo can be ten times that. A short video clip might average 100 megabytes, while a few minutes of high-resolution video can exceed a gigabyte. If you guess the averages, the result inherits the guess. Better inputs come from checking your device or library's own figures: many photo applications report total photo storage and total video storage, and dividing each total by its count gives a real average for your library. Use those measured averages where you can, and treat guessed averages as rough.
Units need the same care as in any storage calculation. The tool works in gigabytes and megabytes, and mixing them is the easiest arithmetic error to make. A thousand megabytes is one gigabyte in the decimal convention the tool uses, so 2,000 photos at 4 megabytes each is 8,000 megabytes, which is 8 gigabytes, not 8,000 gigabytes. Keep every input in the units the tool asks for, and double-check any figure you convert by hand.
A worked example
Suppose you have 2,000 photos averaging 4 megabytes each, 100 videos averaging 200 megabytes each, and 5 gigabytes of other photo-related storage, with a 50-gigabyte plan. Photos contribute 2,000 × 4 = 8,000 megabytes, which is 8 gigabytes. Videos contribute 100 × 200 = 20,000 megabytes, which is 20 gigabytes. Add the 5 gigabytes of other storage, and the total is 8 + 20 + 5 = 33 gigabytes. Against a 50-gigabyte plan, that is 33 ÷ 50 = 66 percent used, leaving 17 gigabytes remaining. The remaining space would hold 17,000 ÷ 4 = 4,250 more photos at the current average size.
Read that result as a story, not just a number. Videos, only 100 of them against 2,000 photos, use 20 gigabytes against the photos' 8. If space gets tight, the videos are the lever: archiving or compressing a handful of videos frees more than deleting hundreds of photos. And the headroom figure, 4,250 more photos, is reassuring only if your photo sizes stay near 4 megabytes. If you start shooting in a higher-resolution format that averages 8 megabytes, the same 17 gigabytes holds only about 2,125 photos, half as many. Re-run the calculation when your habits change, not just when the plan warns you.
A second check guards against the most common input error. If the result looks surprisingly small or large, revisit the averages first. A video average entered as 20 megabytes instead of 200 would understate video storage tenfold, turning a tight plan into a comfortable one on paper. Compare the tool's total with the storage figure your device or service reports for photos; if the two disagree wildly, an input, not the tool, is usually wrong.
Common mistakes
The first mistake is counting photos and ignoring videos. Videos dominate storage in most libraries, and any estimate that omits them is not an estimate at all. The second mistake is using a guessed average for a library whose real average is very different, particularly for libraries mixing phone photos, camera raw files, and screenshots. Measure the average where you can.
The third mistake is forgetting the "other" storage: shared albums, edited versions, and thumbnails or caches that a service counts against the plan. If the tool's total is consistently below the figure your service reports, the gap is usually that other category, and entering a realistic figure for it closes the gap. The fourth mistake is treating the headroom estimate as a deadline. "4,250 more photos" is not a date; at a hundred photos a month it is over three years, and at a thousand a month it is about four months. Divide headroom by your actual shooting rate to turn it into a time, and that time is the figure worth acting on.
Honest limitations
This tool calculates photo and video storage from counts, average sizes, other storage, and a plan size you type in, in your browser. It never accesses an iCloud account, never logs in, never syncs, and never reads your library or measures your real usage. It cannot know your true average sizes, your shooting rate, or what your service counts against your plan, and it assumes future photos resemble past ones in size. The result is a planning estimate, not a measurement, and it should be compared with the figure your device or service reports before any decision such as upgrading a plan or deleting files.
Closing
Count photos and videos separately, use measured averages, and read the result as three answers: used, remaining, and how many more fit. That is enough to plan calmly instead of reacting to a full-storage warning. For the wider picture, total all your storage categories with the iCloud Storage Calculator tool, turn the result into action with the iCloud Storage Cleanup Planner tool, and read our guide How to Optimize Images for Websites to shrink photos before they consume more space.