The disk is a calendar that rewinds

Every surveillance system contains a hidden clock: the day the full disk starts writing over the oldest footage. No alarm sounds when it happens — overwriting is silent, and it is its job. The right question is therefore not "how big is the disk" but how many days of memory it buys: because the event that matters is almost always discovered late — the damage noticed on return, the theft reported by a neighbour — and if the disk's calendar is shorter than the delay, the system recorded everything and kept nothing. This calculator turns cameras, bitrate, disk and hours into days of recording.

The formula: megabits into gigabytes into days

The chain is pure arithmetic. A video stream of 1 Mbps writes 0.45 gigabytes every hour — 3,600 seconds times one megabit divided by eight bits per byte. From there the daily space G = 0.45·n·b·h: cameras times bitrate times hours. The days are the quotient D = 1000·T/G — one thousand gigabytes per terabyte — and the weekly share q says how much of the disk goes every seven days: when it exceeds 100 percent, the disk does not reach Sunday. Three numbers a serious system's specification declares next to the camera points: because the archive is designed, not discovered.

The camera scale: every extra eye shortens the calendar

Bitrate at 4 Mbps, 4-terabyte disk, continuous recording; only the number of cameras grows:

The weekly-share column tells the race: at 16 cameras the disk consumes 121 percent of itself every week — it does not reach Sunday — and the calendar stops at 5.8 days. Same disk, same bitrate: days of recording are not a property of the disk, they are a property of the system.

A complete numerical example

The covered house: 6 cameras at 4 Mbps each — good-quality full HD — a 4-terabyte disk in the recorder, continuous recording. Each camera writes 0.45 × 4 × 24 = 43.2 gigabytes per day; the system together writes 259.2; the disk's 4,000 gigabytes last 4,000/259.2 ≈ 15.4 days, with 45.4 percent of the disk consumed each week. Band b — two weeks of breathing room: the short holiday and the late discovery fit the calendar, and overwriting arrives when the footage no longer matters to anyone.

How to read the four calendar bands

The bands are thresholds of lived life, not of technology. From 30 days up, the full month of archive: even the event reported very late is still on the disk. Between 14 and 30, two weeks of breathing room: the physiology of well-sized home systems. Between 7 and 14, the week is covered: enough for those who check at once, not for those returning from holidays. Below 7 days, the short memory: the disk does not even cover the working week. An arithmetic curiosity of the model: with the chosen steps, the exact thresholds of 30, 14 and 7 days are never touched — it would take a disk that is a multiple of 27 or 63 terabytes, and the scale stops at 24: every system sits on one side of the border or the other, never on it.

The bitrate: the most expensive lever in the bill

The variable b is the only one that appears twice in the project: it decides image quality and, together, the archive's appetite. Compressed full HD lives at 2-4 Mbps; well-made 2K at 6-8; generous 4K asks 10-16 — and from 10 up the calculator flags the camera that drinks the disk: a single continuous 4K view consumes more than 100 gigabytes a day, as much as three full HD cameras together. The serious installers' rule: 4K is reserved for the gates that must read faces and plates, the perimeter lives happily at 4 Mbps — and modern encoding (H.265 instead of H.264) halves the stream at equal image: the declared bitrate is worth more than the camera's name.

Motion detection: the hours you do not record are days you gain

The fourth lever is the most underrated: how many hours a day the system really writes. Continuous recording is the choice of those who want no arguments: 24 hours, everything, always. But the camera on the back seeing two people a day can live on motion detection: the example house, brought from 24 to 8 effective writing hours, goes from 15.4 to 46.3 days — triple the calendar, without touching disk or bitrate. The honest compromise is tiering: continuous on the main gates, motion on the perimeter — and the few seconds of pre-recording modern recorders offer removes motion detection's one historic flaw: the missed first step.

Archive and system: the days are written in the specification

This calculation is the quote's sibling: the system is bought once, the archive is consumed every day. The security camera system cost calculator answers the first question — what points, installation and recorder cost — and this one answers the second: how many days of memory that quote really buys. The two numbers are read together, because this is where quotes differ: the identical recorder with a 2 or an 8-terabyte disk is the same hardware price and a four-times different archive. And the entrance that talks — the video intercom system cost calculator — completes the door trilogy: the voice that opens, the eye that watches, the disk that remembers.

International perspective, limits and correct use

The model is universal by construction: megabits, gigabytes and days carry no passport, and the calendar bands hold in Milan as in Denver. The multi-country perspective sits elsewhere: many jurisdictions limit retention days by law — European data protection rules ask not to keep footage beyond necessity and several authorities indicate 24-72 hours as a baseline, while other markets think in weeks — so the right calendar is the shorter of the technical one and your country's legal one. The calculation does not cover audio streams and metadata (a few percentage points), RAID and redundancy — which divide usable capacity — nor subscription cloud storage. Confirmation belongs to the real recorder: the first day's consumption, read with the system on, is the model's commissioning test.

The bitrate scale: the same house, five appetites

Six cameras, a 4-terabyte disk, continuous recording; only the stream quality changes:

From 30.9 to 3.9 days: the bitrate divides the calendar by eight while the disk stays identical. The 10 Mbps row is the flag's declared threshold — the camera that drinks — and the jump from 8 to 10 costs a day and a half of archive: the move to 4K is never decided by looking only at the monitor, it is decided by looking at this calendar too.

The disk scale: the calendar bought by the terabyte

Same house — six cameras at 4 Mbps continuous — and only the disk growing:

The daily-space column does not move — 259.2 gigabytes, always — because consumption belongs to the system, not the disk. And the days column climbs in exact proportion: double the terabytes, double the calendar — 3.9, 7.7, 15.4, 30.9, 61.7. It is the most honest lever in the bill: linear, predictable, without surprises — and for that reason it should be used last, after bitrate and hours are settled.

The borders that are never touched and the threshold that is

The three calendar borders — 30, 14, 7 days — are declared frontiers but never exactly reachable: with the input steps, an exact border would require a disk that is a multiple of 27 or 63 terabytes, and the scale stops at 24 — the exact line stays empty by arithmetic, not by prudence. The bitrate threshold instead is touched to the millimetre: at exactly 10 Mbps the drinking-camera flag fires — 6.2 days against the 7.7 of the 8 Mbps that stay silent. Two thresholds, two natures: the calendar is grazed, the appetite is declared.

The writing hours: the free lever

The hours scale is the only one that costs nothing. The example house on continuous lives at 15.4 days; brought to 12 effective hours — continuous by day, motion by night — it climbs to 30.9; at 8 effective hours it touches 46.3; and the aggressive 4-effective-hour setup — motion only, dead zones excluded — reaches 92.6 days: a quarter of calendar on the same 4-terabyte disk. Motion detection is not a renunciation: it is the declaration that empty hours do not deserve gigabytes — and with pre-recording on, not even the first step is lost.

The mistakes the specification does not show

The first is the wrong disk: a desktop disk was not born to write around the clock — surveillance-grade ones were, and the difference is discovered on the first scorching August day. The second is RAID counted twice: four 4-terabyte disks in mirrored RAID are 8 usable terabytes, not 16 — the calculation takes the capacity that writes. The third is the nameplate bitrate taken as true: many cameras work in VBR and at night, with noise, write more than by day — real consumption is read on the recorder, not on the box. The last is forgetting the sub-streams: the secondary viewing flows weigh little, but on sixteen cameras little becomes something.

How to buy days without buying terabytes

The order of the levers is the model's final message. First the bitrate: bringing the perimeter from 8 to 4 Mbps doubles those cameras' calendar, and H.265 halves everything once more — perceived quality unchanged. Then the hours: continuous-on-gates and motion-on-perimeter tiering triples the right cameras' days, free. Only at the end the terabytes: linear, honest, but the only ones paid in hardware. And the last lever is not technical: the first-day commissioning test — reading on the recorder what the system wrote in its first twenty-four hours and comparing it with this bill is the fastest way to discover the night camera drinking in silence.

Frequently asked questions

How many days of recording does a 4-terabyte disk hold?

It depends on the system, not the disk: 6 cameras at 4 Mbps continuous write 259.2 gigabytes a day and the 4 terabytes last 15.4 days. With 2 cameras they last 46.3 days; with 16, barely 5.8. The days are a property of the whole system.

How much space does one camera write per day?

The golden rule: 1 Mbps writes 0.45 gigabytes per hour. A full HD camera at 4 Mbps continuous makes 43.2 gigabytes a day; a 4K one at 12 Mbps makes 129.6 — a single generous view consumes as much as three ordinary cameras.

What happens when the disk is full?

The recorder overwrites the oldest footage, silently: it is normal operation, not a fault. That is why the number that matters is the calendar — the days before overwriting — and it is designed for the day you notice events, not the day they happen.

How many days of archive do I really need?

The calculator bands are life thresholds: 30 days for the archive that sleeps well, 14 for home physiology, 7 for frequent checkers. Below 7 days the archive does not cover the working week — and the event discovered after the holidays has already been overwritten.

What bitrate should I set on the cameras?

Compressed full HD lives at 2-4 Mbps, 2K at 6-8, 4K asks 10-16. From 10 Mbps the calculator flags the camera that drinks the disk: 4K is reserved for gates that read faces and plates, the perimeter lives happily at 4. And H.265 encoding halves the stream at equal image.

How much does motion detection stretch the calendar?

In exact proportion to the hours saved: the system at 15.4 days continuous goes to 46.3 with 8 effective writing hours — triple, without touching disk or bitrate. The serious compromise is tiering: continuous on gates, motion on the perimeter, pre-recording on.

What is the weekly disk share?

The percentage of the disk the system consumes every seven days. Below 50 percent the archive breathes; above 100 the disk does not reach Sunday — the quickest way to read an undersized system without doing divisions.

Does RAID change the days of recording?

Yes, downwards: redundancy protects against disk failure but divides usable capacity — a mirrored RAID halves the terabytes that write. Enter the usable capacity, not the sum of disks. And surveillance uses surveillance-grade disks, born to write around the clock.

Why are the 30, 14 and 7-day thresholds never exact?

An arithmetic curiosity of the model: with the input steps, an exact border would require a disk that is a multiple of 27 or 63 terabytes, and the scale stops at 24. The calendar borders stay theoretical: every system sits on one side or the other, never on the line.

Does the law limit retention days?

In many jurisdictions yes: European data protection rules ask not to keep footage beyond necessity — several authorities indicate 24-72 hours as a baseline, extendable with reason — while other markets think in weeks. The right calendar is the shorter of the technical and the legal one in your country.

Does the calculator apply to DVR, NVR and cloud?

The formula is the same wherever a stream is written: megabits times hours times cameras. For cloud the commercial model changes — you pay for the calendar as a subscription — but the daily consumption stays this one. For local DVR and NVR the bill holds net of audio and metadata, a few percentage points.

What does the calculation NOT cover?

Audio streams and metadata, RAID redundancy (enter usable capacity), viewing sub-streams, cloud fees, the physical life of disks. It is the arithmetic bill of the archive: confirmation belongs to the real recorder — the first day's consumption read with the system on is the commissioning test.

Updated July 2026