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Energy Use Intensity (Eui) Calculator

Calculate building Energy Use Intensity (EUI) in kBtu/ft²/yr. Enter annual electricity, gas, fuel oil, and steam usage plus floor area for instant site or source EUI results.

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Energy Use Intensity—kBtu/ft²/yr

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What Is Energy Use Intensity (EUI)?

Energy Use Intensity (EUI) is the standard metric for quantifying a building's annual energy consumption relative to its gross floor area. Expressed in kBtu per square foot per year (kBtu/ft²/yr), EUI enables direct performance comparisons across buildings of varying sizes, types, and climates — an essential capability for facility engineers, portfolio managers, and code compliance officers. A lower EUI signals a more energy-efficient building. According to Penn State EME 807: Technology for Sustainability and Energy Systems, EUI serves as the foundational benchmark in energy auditing and building performance rating programs worldwide.

The EUI Formula

The calculation converts all fuel types to a common unit — the kBtu (thousand British thermal units) — sums them, then divides by floor area:

EUI = Σ (Ei × CFi) / A

  • Ei — annual consumption of each energy source (kWh for electricity, therms for natural gas, gallons for fuel oil, or kBtu for district steam)
  • CFi — site or source conversion factor that translates each fuel's native unit into kBtu
  • A — total gross floor area in square feet, measured inside the exterior walls of the building

Site EUI vs. Source EUI

Two distinct variants exist, and the correct choice depends on the reporting objective:

  • Site EUI measures only the energy delivered to the building's meters — the figure that appears on utility bills. It is the simpler calculation and the preferred metric for operational tracking within a single portfolio.
  • Source EUI applies EPA site-to-source multipliers that account for upstream generation, transmission, and distribution losses. The U.S. EPA specifies the following ratios in ENERGY STAR Portfolio Manager: electricity 2.80, natural gas 1.05, fuel oil #2 1.01, and district steam 1.20. Source EUI runs significantly higher than site EUI for electricity-dominant buildings and is the basis for the ENERGY STAR 1-100 score.

Site Energy Conversion Factors

All fuel inputs must be converted to kBtu before the summation can occur:

  • Electricity: 1 kWh = 3.412 kBtu at the site; multiply the site kBtu total by 2.80 for source energy
  • Natural Gas: 1 therm = 100 kBtu at the site; multiply by 1.05 for source energy. To convert from ccf (hundred cubic feet) to therms, multiply the ccf value by approximately 1.026.
  • Fuel Oil #2: 1 gallon = 138.5 kBtu at the site; multiply by 1.01 for source energy
  • District Steam / Hot Water: Enter directly in kBtu (to convert from pounds of steam, multiply lbs × 1.194); apply a 1.20 source multiplier for source EUI

Worked Calculation Example

Consider a 50,000 ft² office building with annual consumption of 500,000 kWh of electricity and 3,000 therms of natural gas:

  • Site EUI: [(500,000 × 3.412) + (3,000 × 100)] / 50,000 = [1,706,000 + 300,000] / 50,000 = 40.1 kBtu/ft²/yr
  • Source EUI: [(1,706,000 × 2.80) + (300,000 × 1.05)] / 50,000 = [4,776,800 + 315,000] / 50,000 = 101.8 kBtu/ft²/yr

This example illustrates why source EUI is roughly 2.5 times higher than site EUI for buildings with significant electricity consumption — each kWh of electricity carries a 2.80 upstream energy burden at the source level.

EUI Benchmarks by Building Type

The U.S. Energy Information Administration's Commercial Buildings Energy Consumption Survey (CBECS) provides national median EUI values by property type. Typical site EUI ranges include office buildings (50-70 kBtu/ft²/yr), K-12 schools (45-60 kBtu/ft²/yr), retail spaces (55-75 kBtu/ft²/yr), hospitals (200-300 kBtu/ft²/yr), and warehouses (15-30 kBtu/ft²/yr). Buildings that score 75 or above on the ENERGY STAR 1-100 scale qualify for certification. The DOE Energy Intensity Baselining and Tracking Guidance recommends establishing a full 12-month baseline EUI before setting reduction targets, ensuring that seasonal energy patterns are fully captured and that the baseline reflects typical operating conditions.

Interpreting and Applying EUI Results

Once you calculate your building's EUI, context is essential for meaningful interpretation. Climate zone, operating hours, occupancy density, and building age all influence energy consumption patterns. ENERGY STAR Portfolio Manager adjusts raw source EUI for these variables before assigning the 1-100 score, ensuring valid peer comparisons. Tracking EUI trends over multiple years reveals whether efficiency improvements, equipment upgrades, or operational changes have yielded measurable gains. Facilities managers should benchmark their building against national medians and similar-performing peers in their region, then establish realistic reduction targets aligned with their retrofit budget and operational constraints. Regular EUI monitoring — at least annually — enables early detection of equipment degradation or operational drift that might otherwise go unnoticed until utility costs spike significantly.

Reference

Frequently asked questions

What is a good EUI for a commercial office building?
A good site EUI for a commercial office building is generally below 50 kBtu/ft²/yr. The national median for U.S. offices runs approximately 60-70 kBtu/ft²/yr per CBECS data. High-performance certified buildings frequently achieve site EUI values of 30-45 kBtu/ft²/yr. ENERGY STAR-certified offices must score 75 or above on the 1-100 scale. Net-zero energy buildings target an EUI approaching zero by combining deep efficiency retrofits with on-site renewable generation such as rooftop photovoltaics.
What is the difference between site EUI and source EUI?
Site EUI measures only the energy delivered to the building's meters as recorded on utility bills, reflecting actual consumption at the point of use. Source EUI multiplies each fuel by an EPA site-to-source ratio — 2.80 for electricity, 1.05 for natural gas, 1.01 for fuel oil, and 1.20 for district steam — to capture generation and transmission losses upstream of the meter. Source EUI is roughly 2-3 times higher than site EUI for electricity-heavy buildings and is the metric ENERGY STAR Portfolio Manager uses for national benchmarking and certification.
How do I convert natural gas CCF to therms for the EUI calculator?
Multiply the CCF (hundred cubic feet) value from the gas bill by approximately 1.026 to obtain therms. For example, 500 CCF x 1.026 equals 513 therms. Each therm equals 100,000 BTU (100 kBtu), which is the unit this EUI calculator expects for natural gas input. The 1.026 factor reflects the average higher heating value of natural gas, which varies slightly by region and season. Many utility bills list both CCF and therm equivalents; using the printed therm figure directly eliminates rounding error introduced by the conversion.
Why does electricity have a much higher source-to-site ratio (2.80) than natural gas (1.05)?
Electricity carries a 2.80 source-to-site ratio because generating power is thermally inefficient. Approximately 64-68% of the primary energy consumed at power plants is lost as heat during combustion and conversion, with an additional 5-7% lost in transmission and distribution lines before reaching the building. Natural gas travels through pipelines with minimal energy loss, yielding a ratio of just 1.05. This large difference means that reducing electricity consumption lowers source EUI and carbon emissions proportionally far more than cutting an equivalent kBtu of on-site natural gas combustion.
How does EUI relate to the ENERGY STAR 1-100 score?
The ENERGY STAR 1-100 score converts a building's source EUI into a percentile rank compared to similar buildings in the national CBECS dataset. A score of 50 equals the national median; a score of 75 — the minimum threshold for certification — means the building outperforms 75% of its national peers. The EPA adjusts raw source EUI for operating characteristics such as weekly hours, occupancy density, and climate zone before assigning the final score, ensuring fair comparisons across buildings with different usage profiles. EUI is the underlying energy metric; the 1-100 score is its normalized, benchmarked expression.
What floor area should be used when calculating EUI?
EUI calculations require gross floor area, which includes all space measured inside exterior walls — conditioned offices, unconditioned basements, stairwells, mechanical rooms, and parking structures physically attached to the building. Per ENERGY STAR Portfolio Manager guidance, do not subtract columns, elevator shafts, or interior partitions. Using net leasable area instead of gross floor area artificially inflates EUI, making the building appear less efficient than it actually is. Always match the floor area measurement methodology to the benchmark dataset being referenced to ensure a valid apples-to-apples peer comparison.