Disk Growth Estimator
Project when a disk volume will run out of space based on its current growth rate.
About Disk Growth Estimator
By sampling used disk space at two points in time, this calculator derives a linear daily growth rate and projects forward to when the volume will cross an alert threshold (e.g. 90%, matching common Nagios/Prometheus disk-alert configs) and when it will be completely full. Linear extrapolation assumes growth rate stays constant — for volumes with bursty or seasonal growth (e.g. log rotation, batch jobs), treat this as a rough runway estimate and re-sample regularly rather than a hard deadline.
Formula
Days until full
days_until_full = (total_capacity − used_now) / growth_per_day
- T
- total capacity
- U_now
- space used now
- \Delta U / \Delta t
- growth per day from samples
Worked example
With the default values below, Disk Growth Estimator returns Status of Projected to hit the alert threshold in 52.5 days.. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total Capacity (GB)minimum 0
- 500
- Used Space Now (GB)minimum 0
- 300
- Used Space N Days Ago (GB)minimum 0
- 280
- Days Between Samplesminimum 1
- 7
- Alert Threshold (%)accepts 1–100
- 90
Results
- Status
- Projected to hit the alert threshold in 52.5 days.
- Growth Rate (GB/day)
- 2.857
- Current Utilization
- 60.00%
- Days Until Alert Threshold
- 52.5
- Days Until 100% Full
- 70
Step by step
- 1Values used
- 2Days until full
- 3Status= Projected to hit the alert threshold in 52.5 days.
- 4Growth Rate (GB/day)= 2.857
- 5Current Utilization= 60.00
- 6Days Until Alert Threshold= 52.50
- 7Days Until 100% Full= 70
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How many sample points should I use for an accurate forecast?
Two points give a linear trend, which is a reasonable first estimate, but disk growth is often non-linear (bursts from log rotation, batch jobs, or backups). For production capacity planning, sample daily for at least 2-4 weeks and fit a trend line, or feed the series into a monitoring tool's forecast function (e.g. Prometheus predict_linear).
Why alert at 90% instead of 100%?
Filesystems perform worse and become harder to safely clean up as they approach full — ext4's default reserved blocks aside, fragmentation increases and some operations (like large file writes or database checkpoints) can fail unpredictably near capacity. An earlier threshold buys time to add capacity or clean up before an outage.
What if my growth rate is negative?
Negative growth (shrinking usage) means cleanup or log rotation is outpacing new data — the calculator reports 'stable or shrinking' with no full-disk ETA, since linear extrapolation of a negative trend toward zero isn't a meaningful capacity risk.