The only formula you need
Recording time depends on exactly two things: card capacity and recording bitrate. Resolution and frame rate only matter because they change the bitrate your camera uses.
Hours of footage = (capacity in GB × 8000 ÷ bitrate in Mbps) ÷ 3600 × ~0.93 overhead
The handy mental shortcut: 1 Mbps eats about 450 MB per hour. A 100 Mbps 4K mode therefore consumes ~45 GB/hour — which is why that "huge" 256GB card suddenly feels small on a two-day shoot.
Quick reference: hours per card (typical modes)
| Mode | Bitrate | 128GB | 256GB | 512GB |
|---|---|---|---|---|
| Phone 1080p30 | 8 Mbps | 33 h | 66 h | 132 h |
| Phone/action cam 4K30 | 50–60 Mbps | 4.4–5.3 h | 8.8–10.6 h | 17.7–21.2 h |
| GoPro 4K60 | 78 Mbps | 3.4 h | 6.8 h | 13.6 h |
| Mirrorless 4K30 | 100 Mbps | 2.6 h | 5.3 h | 10.6 h |
| Cinema All-I 4K | 400 Mbps | 40 min | 1.3 h | 2.6 h |
| ProRes 422 HQ UHD | 734 Mbps | 22 min | 43 min | 1.4 h |
Planning storage for a real shoot
Producers' rule of thumb: estimate camera hours, convert to GB with this calculator's "shoot time → storage" mode, then multiply by three — one copy on cards, one backup drive on set, one working copy for the edit. Cards themselves should be swapped at natural breaks rather than filled to the brim; recovering a corrupted half-full card is painful, recovering a corrupted full card on day two of a shoot is a disaster story you only tell once.
Choosing bitrates for delivery instead of acquisition? That's the job of our bitrate & file size calculator — and the full format-by-format numbers live in the file size cheat sheet.
FAQ
How much 4K video can a 256GB card hold?
At a typical mirrorless/phone 4K30 bitrate of 100 Mbps, a 256GB card holds roughly 5 hours 15 minutes. At GoPro 4K60 (~78 Mbps) about 6 hours 45 minutes, and at a broadcast-style 50 Mbps about 10 hours 30 minutes. Use the calculator above with your camera’s actual bitrate — "4K" alone does not determine file size, bitrate does.
Why does my card hold less than the math says?
Three reasons: card makers sell decimal gigabytes (256GB = 256 billion bytes) while some tools display binary GiB; the file system reserves a slice of capacity; and cameras stop recording before the card is 100% full. Our calculator already applies a 7% real-world overhead — figures you see here are usable capacity, not marketing capacity.
Does doubling the frame rate double the file size?
Only if the bitrate doubles, which is roughly how cameras behave: most encode 4K60 at about 1.5–2× the bitrate of 4K30. Check your camera’s spec sheet — the calculator’s presets already reflect typical values for each mode.
How much storage do I need for one hour of footage?
Divide the bitrate in Mbps by 2.2 to get GB per hour (e.g. 100 Mbps ≈ 45 GB/hour, 50 Mbps ≈ 22.5 GB/hour, 8 Mbps ≈ 3.6 GB/hour). Switch the calculator to "shoot time → storage" mode to plan a full shoot day, and remember to multiply by 2–3× for backup copies.
Is a bigger card or several smaller cards better?
For professional work, several mid-size cards beat one huge card: a failed 512GB card loses everything, while spreading a shoot across four 128GB cards caps your worst-case loss. Swap cards at natural breaks and back up as you go.