Memory Card Record Time Calculator

Why record-time math never matches the box

Two conversions trip up almost everyone doing this math by hand. The first is bits vs. bytes. Camera manufacturers publish codec data rates in megabits per second (Mbit/s, lower-case b), but storage is sold and consumed in bytes (upper-case B). There are 8 bits in a byte, so a 184 Mbit/s codec is only 23 megabytes per second on disk — miss the ÷8 and every estimate comes out 8× too optimistic. The second is decimal vs. binary gigabytes. Card manufacturers label capacity in decimal units (1 GB = 1,000,000,000 bytes, matching SI prefixes), but your camera's firmware and your computer's file browser often report binary capacity (1 GiB = 1,073,741,824 bytes), which reads roughly 7% smaller for the same physical card. This calculator works entirely in decimal bytes, matching the number printed on the card — the same convention used by the Video File Size Calculator.

On top of that unit gap sits a real, physical one: usable-capacity overhead. A card's marketed capacity is never fully available for footage. The filesystem (exFAT, typically) reserves space for its allocation tables and journal, camera firmware reserves headroom so a card never fills to the exact last byte mid-recording, and some capacity is lost to sector alignment. In practice, usable space runs 85–100% of the label depending on card, filesystem and camera — this tool defaults to a conservative 95% and exposes it as a control so you can tighten or loosen it for a specific card and camera combination you've tested.

The worked example

Take a 128 GB card recording ProRes 422 HQ at 1080p25 (184 Mbit/s nominal), at the default 95% usable factor:

1. Capacity in bytes: 128 × 10⁹ = 128,000,000,000 bytes
2. Usable bytes: 128,000,000,000 × 0.95 = 121,600,000,000 bytes
3. Usable bits: 121,600,000,000 × 8 = 972,800,000,000 bits
4. Bitrate in bits/second: 184 × 10⁶ = 184,000,000 bits/s
5. Record time: 972,800,000,000 ÷ 184,000,000 ≈ 5,287 seconds ≈ 88 minutes (1h 28m)

Every cell in the grid above runs this same five-step chain — only the capacity and the bitrate change. Click or focus any cell to see its own breakdown rendered exactly this way.

Nominal rates are a planning number, not a guarantee

Every bitrate in the preset table is a nominal, constant-ish data rate as published by the manufacturer — real-world VBR (variable bitrate) encoders push more bits at busy, high-detail frames and fewer at static ones, so actual footage runs a few percent either side of nominal. That's normally inside the noise for planning purposes, but on a long day you don't want to run a card down to the wire on a number that's only approximately true. Working assumption on set: budget for about 80% of the computed record time before you card-swap, leaving margin for VBR spikes, in-camera proxies, timecode/metadata tracks and the operator not watching the counter to the second.

Which card class you actually need

Capacity gets all the attention, but for high-bitrate codecs the number that actually stops a shot is the card's sustained write speed, not how many gigabytes it holds. A card can be huge and still choke mid-take if its rating can't keep up with the codec's peak data rate. SD cards use the V-rating (V30/V60/V90 = minimum sustained 30/60/90 MB/s); ProRes 422 HQ at UHD (737 Mbit/s ≈ 92 MB/s) already exceeds V60 and even edges past V90's 90 MB/s guaranteed floor with no headroom to spare, and ProRes 4444 at UHD (1,106 Mbit/s ≈ 138 MB/s) is beyond every SD V-rating. Cameras recording high-bitrate 4K/UHD ProRes or RAW increasingly move to CFexpress (Type A or Type B), which sustains several hundred MB/s — comfortably ahead of any preset here. Always check your card against the write speed, not the marketing read speed printed largest on the package; they are frequently very different numbers.

Pinned codec presets

The ProRes figures are Apple's own published nominal data rates from the Apple ProRes white paper — the industry-standard reference every NLE and camera manufacturer cites for these numbers. A Custom row lets you type any bitrate from 1–5,000 Mbit/s for codecs or camera-specific rates not listed here — this tool intentionally does not invent additional camera-specific presets beyond the white-paper figures.

One footnote worth knowing: adding a PCM 48 kHz/24-bit stereo audio track costs roughly 2.3 Mbit/s on top of the video rate. Across every preset here that's inside the normal VBR variance of the video stream itself, so it's not exposed as a separate control — just know it's there if you're chasing the very last few seconds of a tight estimate.

1. Set the usable-capacity factor (95% by default) so the grid allows for filesystem and firmware overhead.

2. Read across the card capacities and down the codec presets — every cell gives record time in hours and minutes.

3. Click a cell to open its worked byte-to-bitrate arithmetic, or switch to the Video File Size Calculator when you know the runtime instead of the card.