Group room block sizing: how big should your block be?
The size of your room block determines both your attrition risk and your ability to accommodate attendees. Here is the framework for sizing it defensively without leaving rooms on the table.
Key takeaways
- Block sizing should be based on confirmed registrations, not optimistic forecasts.
- Hold back 10-15% of expected count for late additions, not in the block.
- Negotiate "right of first refusal" on rooms above the block — gives flexibility without committing.
- Different event types have different pickup curves; size accordingly.
Hotel room blocks are one of the most over-sized lines on event contracts. Planners block based on optimistic forecasts, fail to fill the block, and pay attrition penalties. Or they under-size and have to find rooms on short notice for late additions. Both errors are preventable with structured sizing.
This post walks through how to size the block right.
Why over-sizing happens
Optimistic forecasts. Planners block based on invitation count, not registration count.
Pressure to "make sure everyone has a room." Leadership pushes for a generous block that accommodates worst-case attendance.
Misunderstanding of how blocks work. Some planners assume rooms can be released freely from the block.
Vendor encouragement. Hotels sometimes encourage larger blocks because they prefer guaranteed rooms.
Why under-sizing happens
Conservative pickup forecasts. Estimating attendance below realistic pickup leaves rooms short.
Tight budget pressure. "Let's not commit to so many rooms."
Fear of attrition. Over-correcting for attrition risk by under-blocking.
The defensive sizing framework
Step 1: Estimate confirmed registrations. Use historical data — invitation acceptance rates from prior similar events.
Step 2: Calculate initial block. Confirmed registrations × pickup factor (typically 0.85-0.95 for B2B events). This is your block.
Step 3: Reserve buffer for late additions. Hold back 10-15% of expected count to be added later via right-of-first-refusal, not in the initial block.
Step 4: Negotiate flexibility. Higher slippage allowance (25-30%), later cutoff dates, re-marketing rights.
Pickup curves by event type
SKO (sales kickoff): strong pickup. 90-95% of registered attendees typically book in the block. Higher initial block.
Conference (cross-functional or external): moderate pickup. 75-85% typical.
Offsite (small leadership): very strong pickup if attendees are required. 95%+.
Optional industry conference: weaker pickup. 60-75% typical. Block conservatively.
Common block sizing mistakes
- Blocking based on invitations. Pickup falls when invitations exceed registrations.
- Ignoring the cutoff date. The cutoff is when attrition is measured. Plan registrations before cutoff.
- Not communicating attrition risk to leadership. When penalties hit, leadership wonders why nobody mentioned this.
- Over-trusting hotel pickup advice. Hotels are biased toward larger blocks.
Quantify your block sizing and attrition risk
Use the Attrition Risk Calculator to size your block defensively against pickup uncertainty.
Open the calculator →Frequently asked questions
What is a typical pickup factor?
Varies by event type and audience. SKOs and required offsites: 90-95%. Optional conferences: 60-75%. Validate against your historical data.
Can I add rooms above the block later?
Often yes via right-of-first-refusal. Negotiate this at brief stage.
What if my block is too small and I cannot get more rooms?
Late additions outside the block typically pay published rate (higher). Build buffer into the block to avoid this.
How does this interact with attrition?
Attrition is the penalty for under-filling the block. Smaller blocks reduce attrition risk; larger blocks accommodate more attendees but increase risk.