Effort Estimation: Rocks, sand and water in planning

Have you ever noticed a team planning extensively, only to deliver much less than expected? Common reasons include: “There were too many supporting tasks,” “Stakeholders added more work,” and of course, “An employee went on vacation” (what a surprise!).

Consider this popular analogy: a jar is filled with large rocks, making it seem full. But when sand is poured in, it fills the gaps between the stones, and suddenly the jar looks even fuller. Add water, and it seeps into the sandy spaces, demonstrating that the jar can hold more than just rocks.

This highlights a common misconception in planning. If the jar’s volume is N, teams often estimate they can deliver large projects totalling N. However, in reality, fitting that many large rocks together is impossible; gaps are created by various factors—stakeholder interactions, integration challenges, rework, and the inevitable surprises that arise.

As Project Managers, we should plan for fewer large rocks than the total volume N. We must also account for the sand: regular flow of support tasks, questions, technical debt, and urgent issues. And let’s not forget water: employees may go on holiday, fall ill, experience productivity slumps, or even leave the organization.

The owner of the Telegram channel “Teamlead Obviousness” shares their approach:

In my team, I allocate about 65% for rocks, 25% for sand, and 10% for water. This ratio has proven to be effective.

While it’s crucial not to copy these exact figures, it’s helpful to understand the general trend: how much sand and water have impacted your team in the past.

For instance, around 6-7 years ago, Skyeng reported that 35% of their workload consisted of rocks, with the remainder being hustle, support, and turnover.

In my experience, risk management plays a significant role in the amount of sand and water poured. For more sensitive projects, the share of “work around the work” tends to increase.

In Conclusion

When estimating effort for tasks, it’s essential to recognize the different types of work involved. It’s wiser to make modest commitments and deliver on time rather than overestimate and later justify delays. Excuses benefit no one—not you when it comes to bonuses, nor those who were waiting for timely results.

References

The work is never just “the work” by Dave Stewart.