How to run a fair random draw

The random part of a draw is the easy part. A good generator gives every entry the same chance in a fraction of a second. Draws go wrong before and after that moment: a list that changed after the closing time, a duplicate entry nobody removed, a rule about ineligible winners invented once the winner turned out to be ineligible, or a result that can’t be shown to anyone who asks. This guide works through a draw in the order you would run it.

Rules for prize draws and giveaways differ by country, and social platforms add their own. This is a practical method, not legal advice; if real money or valuable prizes are involved, check the rules that apply where you and your entrants are.

Write the rules before entries open

Everything that could affect who wins should be decided while nobody knows who will. At a minimum, write down:

• who can enter, including age and location limits;
• when entries open and the exact closing time, with its time zone;
• how many entries each person may have, and whether extra entries can be earned;
• how many prizes there are, what they are, and the order in which they are drawn;
• how and when winners will be told, and how long they have to reply;
• what happens if a winner is ineligible or doesn’t reply.

UK promoters will recognise this list. The advertising self-regulatory code for the UK, the CAP Code, expects prize promotions to state significant conditions such as how to take part, the closing date, any limit on entries, the number and nature of prizes, and how and when winners will be notified. Instagram’s promotion guidelines make whoever runs a promotion responsible for its official rules, its eligibility terms and compliance with the regulations that apply, and require entrants to acknowledge that Instagram doesn’t sponsor or run it.

Freeze and clean the entry list

At the closing time, take a copy of the list and stop changing it. Draw from that copy only. Late entries, corrections and “can you add my friend” all go into a separate note, decided by the rules you published, not into the list.

Then clean it, by the rules rather than by judgement. Remove blank lines and anything that isn’t an entry. Remove entries from people who aren’t eligible. Most importantly, decide what a duplicate is. The random name picker removes repeated names, ignoring capital letters and stray spaces, and shows how many were left out; social handles, email addresses or ticket numbers may make better keys than names when two different people share one.

Duplicates matter more than they look. In a list of 400 lines where one person appears four times, that person has a 4 in 400 (1%) chance of the prize, while everyone else has 1 in 400 (0.25%). Collapse the list to 397 unique people and each has 1 in 397, about 0.252%.

Record the final count. “397 eligible entries after removing 3 duplicates and 2 late entries” is the single most useful sentence in the record.

One entry per person, or weighted entries

Both are fair if the rules say so in advance. With one entry each, every person has the same chance. With weighted entries, such as one per purchase or a bonus entry for a referral, each entry has the same chance and a person’s chance grows with the number they hold: 10 entries out of 200 is a 10 in 200, or 5%, chance of a single prize.

To run a weighted draw with a name picker, list a person once per entry and turn duplicate removal off. With several prizes, decide whether one person can win twice. If not, the rules should say that a person’s other entries are passed over once they win, and the draw should do exactly that.

Fix the prizes and their order

If there are several prizes, decide before the draw which is drawn first. The usual rule is that the first name drawn wins the first prize. Drawing all winners at once, in order, from the entries not yet drawn gives each entry the same overall chance as separate rounds with winners removed: with 250 entries and three prizes, every entry has a 3 in 250 (1.2%) chance of winning one of them.

It also helps to draw reserves at the same moment, in order, under a rule written in advance: if a winner is ineligible or doesn’t reply within the stated time, the prize passes to the first reserve, then the next. That avoids a second public draw and any suspicion that the second draw was steered.

Draw once, on screen, with a timestamp

Use a generator whose method you can name. The tools on this site draw with the browser’s cryptographic generator, crypto.getRandomValues(), and use rejection sampling so that every entry has exactly the same chance. Shuffling the whole list with the list randomizer and taking the top names works the same way; the random number generator suits a numbered list or ticket numbers.

The CAP Code asks for prize draws to be made in accordance with the laws of chance, either by a computer process that produces verifiably random results or by, or under the supervision of, an independent person. Even where no such rule applies, the same idea builds trust: make the draw visible. Run it live or record the screen, show the entry count and the date and time, and keep the device clock correct.

Draw once. Repeating a draw until a preferred name comes up is the one thing that makes an otherwise fair process unfair, and a recording that shows a single draw is the best answer to that suspicion.

Ineligible winners and redraws

Apply the rule you wrote, to the letter. If it says “first reserve”, take the first reserve. If it says “redraw from the remaining entries”, remove the ineligible winner and draw again from the rest, so that in a draw of 250 each remaining entry has a 1 in 249 chance. Don’t remove other entries at the same time, and don’t widen or narrow the eligibility rules after the fact. A redraw that follows a published rule is fair; one improvised after seeing the result is not, even if it happens to pick the same person.

Record and publish what is appropriate

Keep, together:

How much to publish is a separate question from how much to keep. Announcing a winner usually means a first name, a handle or a ticket number, not a full name and town. The CAP Code expects promoters to publish or make available enough to show a valid award took place, ordinarily the surname and county of major prizewinners, while telling entrants in advance and letting them object, and without prejudicing the winners’ privacy. Other places have their own expectations, so take the published rules of your platform and country as the guide.

A short checklist

1. Publish the rules, including the closing time, entry limits, prizes in order and the redraw rule.
2. At the closing time, copy the list and stop changing it.
3. Clean it by the rules and note the final count.
4. Draw winners and reserves in one draw, on screen, with the time showing.
5. Contact winners as the rules say; pass prizes to reserves only under the stated rule.
6. Keep the rules, lists, draw record and recording together.