Work out dram refresh overhead instantly with clear inputs, formula shown and shareable results.
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.
Refresh overhead
overhead = (rows / rows per command) x tRFC / refresh window
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.
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.