Choose a growth model before buying storage
A measured starting point
Measure one current archive, in a consistent unit, before estimating growth. Include the files you intend to keep, such as RAW originals and edited exports. Count extra copies separately. A plan for one library is not a plan for every backup copy.
Percentage growth and fixed additions answer different questions
The calculator compounds your chosen percentage each year. An 800 GB archive growing by 25% becomes 1,000 GB after one year and 1,953.125 GB after four years. If you instead add exactly 200 GB every year, the four-year total is 1,600 GB. Neither number is a prediction: select the model that resembles your own shooting pattern.
Use a small range instead of one confident forecast
Try a low and a high growth assumption. A new camera, more video, or keeping both RAW and JPEG may make an old rate unsuitable. Write down what changed rather than choosing a larger plan simply because it looks economical per gigabyte.
When to act
Compare the projected archive with the space actually available for it, not the label on an already partly used drive. If the current arrangement has enough headroom, keep it and review later. Capacity alone does not establish reliability, recovery, privacy, or upload speed.