How WheelOSpin Selects a Winner

WheelOSpin chooses the result before the visible animation finishes. The animation does not run a second draw and it does not infer the winner from an imprecise physics simulation. That separation lets the selection rule stay simple enough to inspect.

The equal-weight selection path

When weighted mode is off, the current list has n entries. The app requests one unsigned 32-bit value from crypto.getRandomValues, divides that value by 2³² to get a value from zero up to (but not including) one, multiplies by n, and rounds down to an array index.

In simplified form:

index = floor(randomUint32 / 2^32 × numberOfEntries)

The browser Web Crypto API is designed to provide cryptographically strong random values. That is a better source for selection than Math.random(), but the source alone does not turn a browser spinner into a regulated or independently audited draw.

If the number of entries does not divide 2³² evenly, this mapping can assign one more possible 32-bit source value to some indexes than to others. The difference is at most one source value out of more than four billion. It is immaterial for ordinary classroom, game, and decision use, but it is one reason we avoid describing the tool as a certified lottery system.

How the animation reaches the result

After selecting an index, WheelOSpin calculates the start and end angles for that segment. It adds several complete rotations, aligns the chosen segment with the pointer, and adds a small offset that stays inside that segment. Spin speed changes the duration and number of rotations; it does not rerun the draw.

This matters when a wheel appears to stop close to a segment boundary. The displayed winner comes from the selected index, and the animation is constructed to match it. The result modal, history, and statistics all use that same selected entry.

Equal rows mean equal chances

With weighted mode off, every row is one entry. If a name appears twice, it has two chances because it occupies two indexes. Before a fair class or giveaway draw:

1. remove accidental duplicates;
2. remove blank or ineligible entries;
3. decide whether a previous winner remains eligible;
4. keep weighted mode off unless the weighting rule is intentional and disclosed.

The Shuffle button changes display order but does not change the chance attached to an equal-weight row.

Repetition is normal

Independent random selections can repeat. A coin-flip wheel can show Heads five times in a row. A D6 wheel can select 3 twice. Randomness does not promise that a small history will look balanced.

Use Remove winner when you need selection without replacement. After a result, that entry leaves the active list and cannot be chosen again unless it is restored. Do not use that option to imitate a normal coin or die, because physical coins and dice can repeat.

Weighted selection

Weighted mode replaces equal rows with proportional chances. Each positive weight is divided by the sum of all weights. A row with weight 2 is twice as likely as a row with weight 1. The implementation uses a second secure random value and walks through the cumulative weights until the value falls inside one entry’s interval.

See Equal and Weighted Wheel Probabilities for worked examples and the special two-dice distribution.

What WheelOSpin can and cannot establish

WheelOSpin can provide a visible, editable random choice for casual use. It can show the active entries, current weights, recent local history, and the result on one screen.

It does not provide:

• a public seed or a way to reproduce a draw;
• a signed, tamper-evident audit log;
• identity checks for participants;
• protection against someone editing the list before a draw;
• independent certification;
• legal advice about contest or giveaway rules.

For a classroom prompt or game-night choice, those features are usually unnecessary. For money, prizes with legal obligations, regulated gambling, security decisions, or formal public drawings, use a process designed and audited for that purpose.