The Budget Formula: How Many Attendees Will My Spend Actually Buy?

Most budget planning starts with a number someone made up. Flip it: pick the room you want full, and let the math hand you the budget. Every formula below is plug-in - drop in your own numbers as you go.

Step 1 - From attendees to registrations

You can't buy seats. You buy registrations, and a chunk of registrants never show. So the first move is working backwards from the room you want to fill, using a realistic show rate.

• In-person, paid ticket: 50-70% (sunk cost lifts attendance).
• Webinar / virtual: 30-45% - Goldcast's independent 2026 benchmark (6.8M registrations) puts the live show rate near 32%; plan conservatively.
• Lever: multi-touch SMS + personalized email reminders measurably cut no-shows - a well-established mechanism (the same one behind appointment-reminder texts), so layer them onto every run-up.

Step 2 - What each channel buys

Now you know the registration number you have to hit. The next question is what it costs to buy each one - and that depends entirely on the channel.

If you have history, skip the components and use your actual cost per registered for that channel. And mind the units: the public benchmarks in the channel map (LinkedIn CPL ~$408, Sopro 2025) are cost per lead - a completed registration costs more once landing-page conversion is applied. Rule of thumb - and these are planning figures from events run on Premagic, not audited benchmarks: warm audiences (retargeting your own list) land registrations around $40-50; cold paid social runs ~$200 per registration. If you have last event's numbers, they beat any rule of thumb.

Step 3 - The advocacy multiplier

Paid and organic channels each carry a CAC. Advocacy is the channel that bends the math, because it adds registrations on top of that base instead of charging per head. To model it, decide what share of registrations advocacy will drive (target: 30%).

At 30% share that multiplier is 0.30 ÷ 0.70 = 0.429 - advocacy adds ~43% on top of your paid + organic base, at near-zero marginal cost.

This models advocacy top-down, as a lift on your base. The registration forecaster on the advocacy engine builds the same number bottom-up from share rates and visits per share. Use this page to set spend and that one to stress-test the mechanics - expect similar, not identical, numbers.

Step 4 - Blended cost

Those extra advocacy registrations don't just pad the top line - they change what your whole program costs per head. Roll every channel together and you get the number your CFO actually cares about.

Because advocacy is mostly a flat cost (platform + incentives + a person's time), every advocacy registration drags your blended CAC down.

Try it: forecast your event

Here's the worked 1,000-attendee B2B event: attendee target 1,000, show rate 55%, advocacy share 30% - it lands ~1,818 registrations for a blended cost near $22 per registration. All four steps - show rate, channel CAC, the advocacy multiplier, and blended cost - live below. Edit any cell to model your own.

The ROI case: the four numbers your CFO wants

Forecasting is half the job; the other half is proving it worked after the event - the part most event teams struggle with. Advocacy's edge over "buzz" is that it is measurable: every share and every registration ties back to a named participant. Four numbers turn that into a report a CFO signs off on (figures below reuse the worked example - swap in your own):

1. Ad spend avoided. The 545 advocacy registrations would have cost ~$24,500 (warm) to ~$109,000 (cold) on paid social. Against the $8,000 flat program cost, that is ~$16,500-$101,000 the event did not have to spend to hit the same number.
2. Blended CAC, before vs after. Advocacy drags the whole event's cost-per-registration down toward ~$22. Report that delta against your paid-only blended CAC - it is the line a CFO tracks quarter over quarter.
3. Earned media value. Every share is impressions you would otherwise buy. The EMV calculator converts total reach into the ad rate it replaces.
4. Attributed registrations. Per-advocate attribution means you can name which registrations advocacy drove - not a modeled guess. That is the number that answers "how do you know it worked?"

The registration forecaster turns your own inputs into the first two numbers before the event; per-advocate attribution fills in the actual figures after it.

CAC figures here are illustrative planning placeholders - always overwrite with your own historical cost-per-registered before committing budget. Want to see the earned-media value of all that sharing? Try the EMV calculator.