How to calculate LCC: life cycle costing step by step

LCC calculation is the process of bringing every cost a product or building element incurs over the analysis period — purchase, installation, operation, maintenance, replacement and end of life — to a common present value and summing it. The result is a single number: net present cost (NPC). This article builds a life cycle cost analysis compliant with ISO 15686-5 and EN 16627 in seven steps, showing the decision to be taken and the most common mistake at each one. Because the method inherits the module structure of environmental LCA one-to-one, a team that already holds an EN 15804 inventory has the skeleton of its cost model ready.

Step 1: Fix the analysis period and the functional unit

Every LCC starts with a reference study period (RSP) and a functional unit. The analysis period must be identical for all alternatives being compared; it is 50 or 60 years for buildings, 15–25 years for plant equipment and up to 100 years for infrastructure. The functional unit is the denominator of the cost: 1 m² of roof covering, 1 m³ of concrete, 1 km of pipeline. Choosing the same declared unit as the EPD lets the environmental and economic results be read side by side. The most common mistake at this step is comparing alternatives over their own product lives instead of a shared analysis period: a 25-year product and a 40-year product can only be compared within the same period, with replacements and residual value accounted for.

Step 2: Place the cost items in EN 15804 modules

EN 16627 carries the A1-A3, A4-A5, B1-B7, C1-C4 and D module structure of EN 15804 onto the economic axis. Each cost item is assigned to exactly one module, so every row of the environmental table gains a cost row opposite it. The assignment is not arbitrary: the same life cycle event has to land in the same box on both axes, otherwise the cost-versus-carbon comparison loses its meaning.

• A1-A3 — purchase price of materials and products (ex-works)
• A4 — transport to site; A5 — installation labour, equipment and wastage
• B2 — maintenance; B3 — repair; B4 — replacement; B6 — operational energy; B7 — operational water
• C1 — deconstruction; C2 — waste transport; C3 — processing; C4 — disposal
• D — secondary-market value of recovered material; reported separately and never netted into the total

As a second classification axis, each item is given a cost nature: investment (CAPEX), operating (OPEX), end of life and revenue. The module is the standard's primary breakdown; the CAPEX/OPEX split is a second pivot for management reporting. The same item is counted once on each axis; its nature is derived from the module by default and overridden per item where needed (a leased component, for example).

Step 3: Quantity, unit price and the real-versus-nominal decision

Each item is entered as quantity × unit price: 1.05 m² of material including wastage for 1 m² of covering, 0.4 hours of labour, one replacement in year 25. The date of every price is recorded; a quote from two years ago mixed with today's price in the same model makes the result meaningless. Then a single decision is taken: will the model run in real or nominal terms? In a real model cash flows are held at today's prices and a real discount rate is used; in a nominal model each item is inflated with its own price forecast and a nominal rate is applied. Built consistently, the two approaches give the same answer; but the real model removes the general-inflation assumption from the model entirely, which makes it more robust in economies where inflation is high and volatile. The rule is simple: real cash flows are discounted with a real rate, nominal cash flows with a nominal rate, and the two are never mixed.

Step 4: Discount and calculate NPC

Net present cost is every cost discounted from the end of the year in which it occurs and summed: NPC = Σ Cₜ / (1 + r)ᵗ for t from 0 to T. End-of-year discounting is the default convention of ISO 15686-5; a mid-year convention also exists, but what matters is that the same convention is applied to all alternatives. The initial investment (t = 0) is not discounted. If the analysis period exceeds the component's service life, the number of replacements follows N = ceil(T / SL) − 1 and each replacement is discounted in its own year. Service life remaining at the end of the period is converted to a residual value by linear depreciation, discounted as an income in the final year and subtracted from NPC. Without these two mechanisms, long-lived products are systematically penalised.

Step 5: EAC and cost per unit

NPC alone cannot compare different analysis periods. Equivalent annual cost (EAC) converts NPC into a constant annual payment via the annuity factor: EAC = NPC × r / (1 − (1 + r)⁻ᵀ). Within the same analysis period the ranking is identical to NPC; EAC is needed when comparing two studies with different periods or when translating the result into annual budget language. Cost per functional unit (€/m², €/m³) uses the same denominator as the EPD, which makes it the raw material of the cost-versus-carbon comparison: kg CO₂e/m² on one side, €/m² on the other.

Step 6: Worked example — two roof coverings

Two alternatives for 1 m² of roof covering; analysis period 50 years, real discount rate 3%, end-of-year discounting. The figures are illustrative values chosen to show the method and do not represent any product's price.

• Alternative A: installation €80/m², service life 25 years, maintenance €1.5/m²·year, 1 replacement in year 25, disposal €5/m² in year 50
• Alternative B: installation €120/m², service life 40 years, maintenance €1.0/m²·year, 1 replacement in year 40, residual value €90/m² in year 50 (30/40 of life remaining), disposal €5/m²
• Undiscounted total: A €240/m², B €205/m² — B looks cheaper
• Discounted NPC: A ≈ €158/m², B ≈ €163/m² — the ranking reverses
• EAC: A ≈ €6.14/m²·year, B ≈ €6.34/m²·year

The reversal is a matter of discounting geometry. B's biggest advantage, the residual value, arises in year 50 and shrinks from €90 to roughly €20 when discounted over 50 years at 3%; A's disadvantage, the early replacement, occurs in year 25 and shrinks far less. Lower the discount rate to 1% and B moves ahead. This shows that the assumption the result hinges on needs reporting as much as the result itself.

Step 7: Sensitivity and common mistakes

An LCC result is highly sensitive to a handful of parameters: the discount rate, service life, energy price escalation and the largest single cost item. Each is varied by ±25% on its own and the threshold at which the ranking changes is reported; a tornado chart orders the influence of those four parameters at a glance. The mistakes that invalidate a result usually come not from the arithmetic but from inconsistent assumptions:

• Discounting real cash flows with a nominal rate (or the reverse)
• Including VAT in one alternative and excluding it from the other
• Skipping residual value and penalising the long-lived product
• Netting Module D revenue into total NPC
• Combining prices from different dates in one model
• Writing costs into the EPD document — ISO 14025 does not allow it