Resource leveling and smoothing are two complementary schedule optimisation techniques that project managers use to address resource over-allocation — the condition where more work is assigned to a resource during a given time period than the resource has capacity to perform. Resource over-allocation is one of the most common schedule planning failures on complex projects: it produces schedules that look realistic on paper but are undeliverable in practice because they assume resources can work at more than 100% capacity simultaneously. Understanding the difference between leveling and smoothing, and knowing when to apply each, is essential for any project manager building credible, achievable schedules.
What Is Resource Over-Allocation?
Resource over-allocation occurs when a team member, contractor, or piece of equipment is assigned to more work in a given time period than their available capacity permits. In a traditional schedule, over-allocation is invisible unless you explicitly model resource capacity — a Gantt chart shows tasks starting and finishing on their planned dates without revealing that the same developer is assigned to three tasks simultaneously in week 3 when they only have capacity for one. Over-allocation is the primary reason that “technically correct” project schedules frequently produce delivery failures — the schedule passes all logical dependency checks but fails the physical reality check of what resources can actually accomplish.
Resource Leveling: Definition and Mechanics
Resource leveling is a schedule adjustment technique that resolves resource over-allocation by delaying tasks until the assigned resource has the capacity to perform them — even if this means extending the project end date beyond the current baseline. Leveling is the “no constraints on schedule” approach to resource conflict resolution: the project will take as long as it takes to complete all the work without over-allocating any resource at any point in time.
The mechanics of leveling involve identifying all resource over-allocation points, determining which tasks can be delayed without violating mandatory logical dependencies, and delaying those tasks until sufficient resource capacity is available. Critical path tasks that cannot be delayed without extending the project end date may force the end date out to accommodate the resource constraint. Near-critical path tasks with positive float are delayed first, consuming their float before the project end date is extended.
Resource leveling is the appropriate technique when resource availability is the primary constraint — when the organisation cannot provide more resources or hire contractors, and the project end date must flex to accommodate the available capacity. Infrastructure projects, long-duration programmes, and projects with limited specialised resource pools frequently require leveling approaches.
Resource Smoothing: Definition and Mechanics
Resource smoothing is a schedule adjustment technique that optimises resource utilisation within the existing float available on non-critical activities, without extending the project end date. Smoothing is the “no extension of end date” approach: it accepts that some over-allocation may remain if it cannot be resolved without pushing the end date, and prioritises delivering within the committed timeline even if it means some resource over-allocation on non-critical work.
The mechanics of smoothing involve identifying all resource over-allocation points, using available float on non-critical tasks to shift them to periods when the assigned resource has available capacity, and accepting any remaining over-allocation that cannot be resolved within existing float. Unlike leveling, smoothing never delays a task beyond its late finish date — it respects the schedule boundary while optimising resource usage as much as possible within that constraint.
Resource smoothing is appropriate when the project end date is fixed and cannot be extended — contractual delivery dates, market window commitments, regulatory deadlines, and scheduled product launches all create hard date constraints that make smoothing the only viable optimisation approach.
“Resource leveling asks ‘how long will the project take with the resources we have?’ Resource smoothing asks ‘how can we use our resources most efficiently without changing when the project will finish?’ Both are the right question — in different contexts.” — PMI PMBOK Guide
When to Use Each Technique
The choice between leveling and smoothing is fundamentally determined by which constraint is harder — the resource constraint (we cannot add resources) or the schedule constraint (we cannot extend the end date):
- Use leveling when: Resources are fixed and cannot be augmented; the project end date is flexible; quality and sustainability are more important than speed; the project has long duration with complex resource interdependencies.
- Use smoothing when: The end date is contractually fixed or commercially critical; additional resources (overtime, contractors, outsourcing) are available to handle remaining over-allocation; the over-allocation can be managed through workload prioritisation rather than schedule change.
- Use both iteratively when: Complex projects with multiple resource pools and hard schedule constraints require iterative application of both techniques — smooth first within available float, then apply selective leveling to the most critical remaining over-allocations.
Resource Histogram: The Visualisation Tool
A resource histogram plots resource utilisation over time as a bar chart, with a horizontal reference line at 100% capacity. Over-allocated periods appear as bars extending above the capacity line; under-utilised periods appear as shorter bars below it. Resource histograms are the primary visual tool for identifying over-allocation and evaluating the impact of leveling and smoothing interventions — comparing before-and-after histograms shows exactly how scheduling changes affect resource utilisation patterns.
Leveling vs Smoothing Reference
| Dimension | Resource Leveling | Resource Smoothing |
|---|---|---|
| End date impact | May extend project end date | Preserves project end date |
| Float used | Uses all available float plus delays CP | Uses float only; does not delay CP |
| Over-allocation resolved? | Fully resolves all over-allocation | May leave some over-allocation |
| Primary constraint | Resources are the binding constraint | Date is the binding constraint |
| Suitable when | End date is flexible | End date is fixed/contractual |
Key Takeaways
- Resource over-allocation is one of the most common causes of schedule failures — schedules that look valid on dependency logic may be undeliverable if resources are simultaneously assigned to more work than their capacity allows.
- Resource leveling resolves all over-allocation by delaying tasks — including potentially extending the project end date — making it appropriate when resources are the binding constraint.
- Resource smoothing optimises resource utilisation within available float without extending the end date — making it appropriate when the project end date is fixed and cannot move.
- The choice between leveling and smoothing depends on which constraint is harder: resource availability or schedule date — identifying the harder constraint is the first resource optimisation decision.
- Resource histograms visualise over-allocation clearly and enable before-and-after comparison of the impact of leveling and smoothing interventions on resource utilisation patterns.
- In practice, complex projects require iterative application of both techniques — smooth within available float first, then apply selective leveling to the most critical remaining over-allocations.
