SPIES Estimate

Step 1 Generate bins

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Enter the name of the value you are estimating. Bins are just a list of values — use Minimum/Maximum/Bin size below to generate an evenly-spaced list (e.g. weeks 0–8), or pick a preset like story points on the left for a non-uniform list.

Value name

Minimum

Maximum

Bin size

Step 2 Choose probabilities for each bin

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Set the sliders below to assign points to each bin. The higher the likelihood that the actual value falls within a bin, the more points you should give it. Points are automatically normalized to a 100% likelihood total.

	Points	Likelihood

Total points: — normalized to 100%

Step 3 See results, define confidence threshold

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Peak-outward (original SPIES method)

Starts at the highest-probability bin and grows toward whichever neighbor has more mass, until it reaches the target confidence. Two-sided — answers "what's the narrowest range with an X% chance of containing the true value?". The general-purpose method from the original paper — works for any estimate.

Cumulative (left → right)

Accumulates probability from the lowest bin upward until it hits the target confidence. Helps with one-sided estimates — answers "by when / what value am I X% confident this is done?". Good for durations/deadlines. Added in this tool as a practical shortcut — not part of the original paper.

SPIES Probability Distribution

Confidence

Shareable preset URL (loads these bins & points on open)