Sub-capacity licensing is the IBM model that lets a customer license a PVU-metered product against only the virtual cores assigned to it, instead of every physical core in the server, which on a 64-core host running a four-vCPU workload cuts the required PVUs from roughly 4,480 to 280, a saving of about 94 percent. The right to pay at sub-capacity is conditional, not automatic, and the condition is a correctly deployed IBM License Metric Tool. Fail the condition and IBM reverts the entire eligible estate to full-capacity billing.
How sub-capacity works
Under full capacity, an IBM product is licensed for every activated core on the physical machine, regardless of how little of that machine the product touches. Sub-capacity instead counts the cores allocated to the virtual machine or partition where the product runs, applies the PVU rating to that smaller number, and licenses the peak measured over the quarter.
| Scenario | Cores licensed | PVUs at 70 per core |
|---|---|---|
| Full capacity, 64-core host | 64 | 4,480 |
| Sub-capacity, 8-vCPU VM | 8 | 560 |
| Sub-capacity, 4-vCPU VM | 4 | 280 |
| Sub-capacity, 2-vCPU VM | 2 | 140 |
The saving scales with how lightly the IBM product sits on the host. The denser the virtualization and the smaller the IBM footprint on each large server, the larger the sub-capacity advantage. The product-level mechanics are set out in IBM sub-capacity licensing.
The ILMT condition
Sub-capacity is not a pricing choice the buyer simply elects. It is an entitlement that depends on proving virtual capacity with the IBM License Metric Tool. ILMT must be installed within 90 days of first eligibility, scan the relevant servers at least every 30 minutes, and produce quarterly reports retained for two years.
Compliance warning: Sub-capacity is the highest-value and highest-risk IBM model at once. The 94 percent saving on a single host becomes a 94 percent exposure if ILMT cannot prove it during an audit. IBM auditors routinely target sub-capacity claims first, because each unproven claim reverts to full capacity and the reversal compounds across every PVU-metered product on the host. Treat ILMT coverage as the asset that protects the saving, and reconcile it every quarter.
Why it matters to buyers
Sub-capacity is where the largest IBM savings and the largest IBM audit findings both live, which makes it the most consequential concept in an IBM estate. A buyer who virtualizes aggressively and proves it with ILMT pays a fraction of the full-capacity bill. A buyer who virtualizes and lets ILMT coverage drift carries the full-capacity liability without the saving. The disciplined approach is to design the estate for sub-capacity, then defend the claim with continuous ILMT evidence. Our IBM advisory practice validates sub-capacity positions before audit, and the full context sits in the IBM licensing complete guide and IBM license audit.
Eligible virtualization technologies
Sub-capacity licensing is available only on virtualization technologies IBM has approved as eligible, and the eligibility list is specific. Common platforms such as VMware ESXi, IBM PowerVM with LPARs, Microsoft Hyper-V, and major public cloud instance types are recognized, each with its own rules for how the cores assigned to a virtual machine are counted. Technologies that are not on the eligible list, or eligible technologies configured outside the approved boundaries, revert the product to full-capacity licensing regardless of how the workload is actually constrained. The capping method also varies by platform, so the cores counted for a VMware virtual machine follow different rules than those for a Power LPAR or a cloud instance, and the measurement must reflect the correct method for the platform in use. Buyers planning a virtualization or cloud migration should confirm sub-capacity eligibility and the applicable counting rules before they move IBM workloads, because a migration onto an ineligible or misconfigured platform can quietly convert a sub-capacity saving into a full-capacity liability. The platform-specific rules are tracked as part of the IBM advisory work and feed directly into the audit position.
Managing sub-capacity over time
Sub-capacity is not a position a buyer sets once and forgets, because the estate it depends on keeps changing. Every new host, every migrated virtual machine, and every product upgrade can alter what must be measured and whether the existing measurement still proves the claim. The discipline is to treat sub-capacity as a continuous control: reconcile ILMT coverage each quarter, confirm that new platforms remain on the eligible list, and verify that the cores assigned to each IBM workload still match what the reports show. Buyers who run this cycle keep the saving and the evidence aligned, while buyers who let the estate drift away from the last clean measurement carry an exposure they cannot see until an audit reveals it.