DRAM Refresh Overhead Calculator
Work out dram refresh overhead instantly with clear inputs, formula shown and shareable results.
About DRAM Refresh Overhead Calculator
DRAM cells leak, so every row must be refreshed within a 64 ms window (32 ms above 85 degrees). Refresh overhead is the total refresh time divided by that window — a few percent today, but it grows with density because row count rises while the window does not.
Formula
Refresh overhead
overhead = (rows / rows per command) x tRFC / refresh window
Worked example
With the default values below, DRAM Refresh Overhead Calculator returns Refresh overhead of 4.4800%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Rows per bank
- 65,536 rows
- Refresh window
- 64 ms
- Refresh cycle time
- 350 ns
- Banks
- 16 banks
Results
- Refresh overhead
- 4.4800% %
- Refresh commands per window
- 8,192 commands
- Time spent refreshing per window
- 2.87 ms
- Interval between refreshes
- 7.81 µs
Step by step
- 1Values used
- 2Refresh overhead
- 3Refresh overhead= 4.4800 %
- 4Refresh commands per window= 8,192 commands
- 5Time spent refreshing per window= 2.87 ms
- 6Interval between refreshes= 7.81 µs
Assumptions and limits
- The result is derived only from the 4 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 4 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
Why does refresh overhead grow with capacity?
More rows must fit into the same 64 ms window, so either more commands or longer commands are needed. At very high densities this becomes a serious bandwidth tax.
Why halve the window when hot?
Leakage roughly doubles every 10 degrees, so above 85 degrees the retention time falls below 64 ms and the controller must refresh twice as often.
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