About the EUI Calculator – Energy Use Intensity
Energy Use Intensity (EUI) is the standard measure of how much energy a building consumes relative to its size. It is calculated by adding up a building's total annual energy consumption from all fuel sources — converted to a common unit, thousands of British thermal units (kBtu) — and dividing by the building's gross floor area in square feet. The result, expressed in kBtu per square foot per year (kBtu/ft²/yr), lets you compare a small office to a large warehouse on equal footing, or track the same building's performance from year to year.
The Site EUI formula
Site EUI is the metric used by ENERGY STAR Portfolio Manager, ASHRAE, and most building energy codes:
- Site EUI (kBtu/ft²/yr) = Total annual site energy (kBtu) ÷ Gross floor area (ft²)
- Electricity is converted at 3.412 kBtu per kWh, the standard energy-equivalence factor.
- Natural gas is converted at 100 kBtu per therm, since one therm is defined as exactly 100,000 Btu.
- Any other metered fuel (fuel oil, propane, district steam) is entered directly in kBtu and added to the total.
Site EUI vs. Source EUI
Site EUI only counts the energy delivered to the building, as read on utility meters. Source EUI goes one step further and accounts for the energy lost generating and transmitting that delivered energy — the fuel burned at a power plant, minus transmission losses, before electricity ever reaches the building. This calculator applies the national average site-to-source ratios published by ENERGY STAR Portfolio Manager: 2.8 for grid electricity and 1.05 for natural gas. Because generating and delivering electricity is far less efficient than burning gas on-site, Source EUI is almost always higher than Site EUI, and buildings that rely heavily on electricity see a bigger gap between the two figures.
How to get accurate results
- Pull electricity (kWh) and natural gas (therms) totals straight from twelve consecutive months of utility bills — mixing partial years distorts the result.
- Use gross floor area, not rentable or conditioned area — EUI benchmarking programs consistently define floor area as the total enclosed space, including mechanical rooms and common areas.
- If the building uses fuel oil, propane, or district steam/chilled water, convert it to kBtu first and enter it as "Other fuel use."
- Compare your EUI only against buildings of a similar type, climate zone, and operating hours — a hospital and a warehouse have very different baselines even at the same floor area.
Practical context
EUI is a starting point for benchmarking, not a pass/fail grade on its own. A high EUI can reflect genuine inefficiency, but it can also reflect a building type that legitimately uses more energy per square foot — a data center, hospital, or restaurant will always show a much higher EUI than an office or warehouse. Use this calculator to compute your own building's numbers, then compare them against ENERGY STAR's building-type-specific benchmarks or your building's own EUI history over time.