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BigQuery Slot Estimate Calculator

Work out bigquery slot estimate instantly with clear inputs, formula shown and shareable results.

About BigQuery Slot Estimate Calculator

A slot is a unit of query processing capacity, and reservations are purchased in blocks of 100. Peak requirement is concurrency times slots per query, but average utilisation is far lower because peaks are short — which is why on-demand or autoscaling reservations frequently beat a fixed reservation sized for peak.

Formula

Slot sizing

peak slots = concurrent queries x slots per query; reservation = round up to the next 100; utilisation = peak x peak hours / 24 / reservation

Worked example

With the default values below, BigQuery Slot Estimate Calculator returns Slots required at peak of 1,440. Change any input above and every figure here is recalculated in your browser.

Inputs used

Concurrent queriesminimum 1
12
Average slots per queryminimum 1
120
Hours per day at peak concurrencyaccepts 0.1–24 h
6 h
Price per slot-hourminimum 0
0.04

Results

Slots required at peak
1,440
Reservation size rounded to a purchasable block
1,500 slots
Monthly reservation cost
$43,800.00
Average slot utilisation
24.0%

Step by step

  1. 1Values used
  2. 2Slot sizing
  3. 3Slots required at peak= 1,440
  4. 4Reservation size rounded to a purchasable block= 1,500 slots
  5. 5Monthly reservation cost= 43,800.00
  6. 6Average slot utilisation= 24.0

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 2 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

When is a fixed reservation cheaper than on-demand?

When utilisation is high and sustained. At 25 percent average utilisation, on-demand scan pricing or autoscaling reservations are usually cheaper.

What happens when queries exceed the reservation?

They queue rather than fail, sharing available slots. Query duration stretches, which is acceptable for batch work and not for interactive dashboards.

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