Degree days came from the need to give a rating of the demand for fuel in relation to the outside temperature. The calculation of degree days is what allows your home fuel oil delivery service to accurately predict when your tank is low and you need a fuel oil delivery.

It's actually a simple mathematical formula. Add the high and low temperature for any given day and divide that by two. If the number you come up with is over 65, then that days heating degree days is equal to zero.

If the number you come up with is under 65, subtract it from 65 and that number is that particular days heating degree days. For instance. If yesterdays high was 50 degrees, and the low was 20 degrees, you you add 50 plus 20 = 70 divided by 2 = 35 -- 65 minus 35 = 30 degree days for yesterday.

Heating Degree Days – 30 Years (Boston)

Year	Sep	Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Total
1973-74	94	289	570	782	1,023	1,000	809	429	335	5,331
1974-75	102	458	587	836	925	918	866	590	162	5,444
1975-76	70	239	395	941	1,198	800	733	331	166	4,873
1976-77	55	393	688	1,108	1,290	956	623	414	158	5,685
1977-78	85	304	498	948	1,127	1,057	885	480	209	5,593
1978-79	150	381	635	916	1,002	1,169	691	481	149	5,574
1979-80	80	390	484	873	1096	1071	866	481	185	5,526
1980-81	72	387	706	1120	1344	794	796	393	200	5,812
1981-82	91	419	628	979	1300	948	811	496	231	5,903
1982-83	71	338	515	783	1040	896	749	478	223	5,093
1983-84	42	327	561	1012	1182	790	1020	563	240	5,737
1984-85	142	359	605	781	1255	897	758	471	204	5,472
1985-86	65	298	580	1035	1036	1008	746	490	258	5,516
1986-87	98	344	674	904	1112	997	814	588	285	5,816
1987-88	57	326	626	888	1145	945	792	541	253	5,573
1988-89	64	443	541	992	938	959	853	565	193	5,548
1989-90	88	294	660	1336	880	857	762	524	307	5,708
1990-91	84	236	496	744	1096	803	721	407	126	4,713
1991-92	111	278	586	890	1049	924	911	538	325	5,612
1992-93	109	386	656	930	1002	1056	880	493	167	5,679
1993-94	89	387	576	946	1320	1059	827	404	226	5,834
1994-95	61	288	479	813	932	1016	814	561	258	5,222
1995-96	113	214	685	1032	1074	981	875	510	264	5,748
1996-97	51	358	739	792	1,104	831	878	551	269	5,522
1997-98	84	383	693	917	955	826	736	462	192	5,164
1998-99	36	321	606	797	1092	872	790	468	215	5,197
1999-00	39	363	505	851	1152	887	665	523	250	5,235
2000-01	108	341	628	1103	1079	924	914	488	217	5,802
2001-2002	67	286	492	753	867	798	764	458	231	4716
2002-2003	30	407	653	980	1,264	1,077	840	614	301	6,166
2003-2004	27	365	565	891	1,369	929	784	458	210	5,598
Most Recent Year Versus 30-Year Norm	-166%	5%	-3%	-1%	19%	0%	0%	-6%	-6%	2%
30-Year Norm	72	349	583	899	1,103	932	784	484	222	5,502
Monthly Percentage	1%	6%	11%	17%	20%	17%	15%	9%	4%	100%
Avg monthly oil usage based on 900 gallons	9	54	99	153	180	153	135	81	36	900