1. The short answer
Heating degree days measure how much heating a period actually demanded, not how cold it felt, by summing the gap between a reference indoor temperature and the outside daily average over every day cold enough to count as a heating day. The figure only means anything next to its own institution's base temperature: DWD and VDI 3807 use a 15°C heating-day threshold and a 20°C reference for Germany, Eurostat uses a 15°C threshold with an 18°C reference across the EU, the EEA simplifies the whole continent to one 15.5°C figure for its map, and the US EIA works from 65°F with no separate threshold day. None of these is more correct than another; they are different institutional conventions, and this page keeps them side by side instead of picking one.
The practical use is the one our partial fill and order size entry already leans on: scaling an expected delivery from last year's consumption and the ratio of this period's heating degree days to the same period a year ago.
2. How the number is built, and who sets the rule
A weather service or statistics office fixes a base temperature once, and a daily temperature record does the rest of the work automatically. In Germany, the Deutscher Wetterdienst publishes the VDI 3807 series from its own station network; Eurostat aggregates member states' station data into the harmonised nrg_chdd series; the EEA takes a further simplification step to produce one figure comparable across the whole European map; and in the United States, the EIA calculates HDD from the daily high and low reported by weather stations, with no separate European-style threshold day.
None of these bodies sets a policy target with the number, unlike a subsidy rate or a tax rebate: heating degree days are a measurement convention, not a regulation, so there is no legal status to check here, only a method to read correctly. A degree day figure also says nothing about how much heat a litre of fuel releases once burned, a separate property covered in our calorific value entry.
3. Four institutions, four base temperatures
The table below places all four methods side by side. Do not average them or swap one figure for another: a Eurostat degree day and a DWD degree day are built from different reference temperatures and are not the same unit of demand.
| Institution / region | Heating-day threshold | Reference temperature | Formula | Status date | Source |
|---|---|---|---|---|---|
| DWD / VDI 3807 (Germany) | Daily mean below 15°C | 20°C | HDD = 20°C minus daily mean, summed in kelvin-days | Dataset description, accessed 18 September 2026 | DWD Opendata |
| Eurostat, nrg_chdd (EU) | Daily mean at or below 15°C | 18°C | HDD = sum of (18°C minus daily mean); zero on warmer days | Metadata updated 25 October 2024 | Eurostat nrg_chdd_esms |
| EEA (EU map, 1950 to 2100) | Not separately defined | 15.5°C, one simplified figure | Same subtraction principle, one EU-wide base | Figure last modified 11 September 2024 | EEA |
| EIA (United States) | None; negative days count as zero | 65°F (about 18.3°C) | HDD = 65°F minus daily mean from daily high/low; negatives set to zero | Glossary entry, accessed 18 September 2026 | EIA Glossary |
Context for why this matters right now: retail heating oil in Germany averaged 181.0 cents per litre on 17 September 2026, up 90.3% on the year, according to Tecson, so the litres a degree day figure tells you to expect carry real cost, tracked in our heating oil price entry.
4. Why the bases differ, and why that is not an error
The four institutions did not make a mistake by disagreeing; each base temperature reflects a different assumption about buildings and a different job the number has to do. DWD's 20°C reference reflects a German indoor comfort assumption used since VDI 3807 was adopted; Eurostat's 18°C reference is built for aggregating across EU member states with varied building stock; the EEA's single 15.5°C basis trades precision for a map that can compare Portugal and Finland on one scale; and the EIA's 65°F convention long predates the metric alternatives and is tied to decades of US utility and energy-market practice.
This research could not find a single sourced typical annual heating degree day figure for Germany, France, Spain, the UK or the US Northeast, the markets this site covers, with a primary source and a year attached, so none is stated here. A cold spell that pushes heating degree days up fast can collide with two supply-side pressures covered elsewhere on this site: distillate stocks running low, tracked in our diesel shortage entry. Several European markets also switch diesel to its winter cold-weather grade on a fixed autumn date, covered in our winter diesel entry. Neither changes the heating degree day formula itself, but both can change how quickly a supplier can actually deliver the litres the formula says you need.
5. What this means for your delivery: will a partial fill last until January?
Take last winter's total consumption in litres, divide it by last winter's heating degree day total from your local weather station, and you get a litres-per-degree-day figure you can apply to any period. Suppliers use the same idea in reverse to scale an expected delivery: expected litres are roughly last year's consumption multiplied by the ratio of this period's heating degree days to the same period last year, at the same station.
Worked example, with illustrative round numbers, not measured data: say your household burned 2,000 litres last heating season, and the DWD-method heating degree day total for that same season at your nearest station was 2,200 kelvin-days. That gives 2,000 divided by 2,200, or about 0.91 litres per degree day. If the period between now and the end of January is forecast or measured so far at 850 degree days on the same basis, your estimated need for that period is 0.91 multiplied by 850, or about 773 litres. Swap in your own station's figures from DWD Opendata, the Eurostat databrowser or your national weather service, and the same three steps work for any period.
To judge whether a partial order will last until January, add your current tank level to the partial quantity you are considering, and compare that total against the estimated litres your own ratio predicts for the delivery date to end-of-January window. If the total covers that estimate with a reasonable margin and your supplier's lead time is short, a partial fill is a reasonable bet. If it falls short, or your supplier's lead time runs to weeks rather than days, ordering closer to a full tank now removes the risk of a second, possibly costlier, order landing mid-winter. Converting any litre figure into a number of heating days at your household's own burn rate is covered in our days of supply entry, and our heating oil, buy now or wait page weighs that timing question against current market conditions. Put your own numbers into our fuel cost calculator to run this comparison for your own bill.
Turn your own consumption and degree days into a delivery estimate.
Run the calculator6. What heating degree days do not tell you
A heating degree day figure is a demand signal built from temperature alone; it is not a price forecast, not a weather forecast, and not the same number as the energy content of your fuel. This page and the sources behind it make no claim about where oil or gas prices are headed, only about how much colder or warmer a period has been than a chosen reference. Confusing the DWD's 20°C-based German figure with Eurostat's 18°C-based EU figure is the single most common misreading found while researching this page: the two use different reference temperatures and are not interchangeable without conversion, and neither converts directly into the US EIA's Fahrenheit-based figure without a unit change.
A second mix-up is assuming a single official typical annual heating degree day total exists for your country; this page found none with a primary source and a year attached for Germany, France, Spain, the UK or the US Northeast, and states that gap rather than repeating an unsourced number from a secondary site. A third is treating your estimated litre need as the same question as how long oil already in your tank stays usable once delivered; that shelf life question is covered separately in our heating oil shelf life entry.