Dev Patel as Srinivasa Ramanujan

Jeremy Irons as G. H. Hardy

Toby Jones as Littlewood

Devika Bhise as Janaki

Stephen Fry as Sir Francis Spring

Kevin McNally as Major MacMahon

Jeremy Northam as Bertrand Russell

Anthony Calf as Howard

Pádraic Delaney as Beglan

Shazad Latif as Chandra Mahalanobis

Roger Narayan as Scribe

Arundathi Nag as Komalatammal

Dhritiman Chatterjee as Narayana Iyer

Richard Johnson as Vice Master Henry Jackson

Raghuvir Joshi as Narasimha

Malcolm Sinclair as Professor Cartwright

San Shella as Dr. Muthu

Richard Cunningham as Hobson

Thomas Bewley as Baker

Pip Barclay as Student

David Shaw-Parker as Porter

Dominic Cazenove as Waiter

Nicholas Agnew as Andrew Hartley

Alan Booty as Grocer

Shenagh Govan as Postal Worker

Alexander Forsyth as Cadet

Enzo Cilenti as Doctor

Elaine Caulfield as Hospital Nurse

Alex Bartram as Hospital Doctor

Christopher Ravenscroft as J. J. Thompson

Imogen Sage as Nursie (uncredited)