| I | Island; Ireland, used chiefly of creations of titles of honour of; 'The First' in a sequence of I, II, Ill etc |
| i/c | in charge; in command , |
| IA | Indian Army; Iowa |
| IAM | Institute of Advanced Motorists; Institute of Aviation Medicine |
| IAPS | Incorporated Association of Preparatory Schools |
| IARO | Indian Army Reserve of Officers |
| IASC | International Arctic Science Committee |
| IATA | International Air Transport Association |
| IBA | Independent Broadcasting Authority; International Bar Association |
| IBRD | International Bank for Reconstruction and Development (World Bank) |
| ICA | Institute of Contemporary Arts; Institute of Chartered Accountants in England and Wales |
| ICAO | International Civil Aviation Organization |
| ICC | International Chamber of Commerce |
| ICD | Iuris Canonici Doctor (Doctor of Canon Law); Independence Commemorative Decoration (Rhodesia) |
| ICE | Institution of Civil Engineers |
| ICFTU | International Confederation of Free Trade Unions |
| IChemE | Institution of Chemical Engineers |
| ICI | Imperial Chemical Industries |
| ICL | International Computers Ltd |
| ICM | International Confederation of Midwives |
| ICOMOS | International Council of Monuments and Sites |
| ICRC | International Committee of the Red Cross |
| ICRF | Imperial Cancer Research Fund |
| ICS | Indian Civil Service |
| ICSTIS | Independent Committee for Supervision of Telephone Information Services |
| ICSTM | Imperial College of Science, Technology and Medicine, London |
| ID | Independence Decoration (Rhodesia); Idaho |
| IDA | International Development Association |
| idc | completed a course at, or served for a year on the Staff of, the Imperial Defence College |
| IEA | Institute of Economic Affairs |
| IEE | Institution of Electrical Engineers |
| IEng | Incorporated Engineer |
| IFS | Irish Free State; Indian Forest Service |
| IG | Instructor in Gunnery |
| IISS | International Institute of Strategic Studies |
| ILEA | Inner London Education Authority |
| ILP | Independent Labour Party |
| ILR | Independent Local Radio; International Labour Review |
| IM | Individual Merit |
| IMA | International Music Association; Institute of Mathematics and its Applications |
| IMCB | International Management Centre from Buckingham |
| IME | Institute of Medical Ethics |
| IMechE | Institution of Mechanical Engineers |
| IMEDE | Institut pour l'Etude des Methodes de Direction de I'Entreprise |
| IMF | International Monetary Fund |
| IMinE | Institution of Mining Engineers |
| Imp | Imperial |
| IMS | Indian Medical Service; Institute of Management Services; International Military Staff |
| IN | Indian Navy; Indiana |
| Inc | Incorporated |
| Ind | Independent |
| Industs | Industries |
| Inf | Infantry; Information |
| Inspr | Inspector |
| Inst | Instructor; Institute; Institution |
| Instr | Instructor |
| insur | insurance |
| Intell | Intelligence |
| Internat | International |
| IOD | Institute of Directors |
| IoM | Isle of Man |
| IoW | Isle of Wight |
| IPA | International Publishers' Association |
| IPCS | Institution of Professional Civil Servants |
| IPlantE | Institution of Plant Engineers |
| IPM | Institute of Personnel Management |
| IPPR | Institute for Public Policy Research |
| IPPS | Institute of Physics and The Physical Society |
| IProdE | Institution of Production Engineers |
| IPS | Indian Police Service; Indian Political Service; Institute of Purchasing and Supply |
| IPU | Inter-Parliamentary Union |
| IRA | Irish Republican Army |
| IRRV | (Fellow/Member of) Institute of Revenues, Rating and Valuation |
| IS | International Society of Sculptors, Painters and Gravers |
| ISC | Indian Staff Corps; Imperial Service College, Haileybury |
| ISIS | Independent Schools Information Service |
| ISM | Incorporated Society of Musicians |
| ISO | Imperial Service Order; International Organization for Standardization |
| IT | Information Technology; Indian Territory (US) |
| ITA | Independent Television Authority (later IBA) |
| ITB | Industry Training Board |
| ITC | International Trade Centre; Independent Television Commission |
| ITN | Independent Television News |
| ITV | Independent Television |
| IUCN | World Conservation Union (formerly International Union for the Conservation of Nature and Natural Resources) |
| IVS | International Voluntary Service |
| IWEM | Institution of Water and Environmental Management |
| IY | Imperial Yeomanry |