v3.26.1
Cover
 12 Months Ended
Mar. 31, 2026
shares
Entity Information [Line Items]  
Document Type 20-F
Document Registration Statement false
Document Annual Report true
Document Period End Date Mar. 31, 2026
Current Fiscal Year End Date --03-31
Document Transition Report false
Document Shell Company Report false
Entity File Number 001-14958
Entity Registrant Name NATIONAL GRID PLC
Entity Incorporation, State or Country Code X0
Entity Address, Address Line One 1-3 Strand
Entity Address, City or Town London
Entity Address, Postal Zip Code WC2N 5EH
Entity Address, Country GB
Entity Common Stock, Shares Outstanding 5,198,968,690
Entity Well-known Seasoned Issuer Yes
Entity Voluntary Filers No
Entity Current Reporting Status Yes
Entity Interactive Data Current Yes
Entity Filer Category Large Accelerated Filer
Entity Emerging Growth Company false
ICFR Auditor Attestation Flag true
Document Financial Statement Error Correction [Flag] false
Document Accounting Standard International Financial Reporting Standards
Entity Shell Company false
Amendment Flag false
Document Fiscal Period Focus FY
Document Fiscal Year Focus 2026
Entity Central Index Key 0001004315
Ordinary shares  
Entity Information [Line Items]  
Title of 12(b) Security Ordinary Shares of 12 204/473 pence each
Trading Symbol NG
Security Exchange Name NYSE
American Depositary Shares  
Entity Information [Line Items]  
Title of 12(b) Security American Depositary Shares, each representing five
Trading Symbol NGG
Security Exchange Name NYSE
5.602% Notes Due 2028  
Entity Information [Line Items]  
Title of 12(b) Security 5.602% Notes due 2028
Trading Symbol NGG28
Security Exchange Name NYSE
5.809% Notes Due 2033  
Entity Information [Line Items]  
Title of 12(b) Security 5.809% Notes due 2033
Trading Symbol NGG33
Security Exchange Name NYSE
5.418% Notes Due 2034  
Entity Information [Line Items]  
Title of 12(b) Security 5.418% Notes due 2034
Trading Symbol NGG34
Security Exchange Name NYSE
Business Contact  
Entity Information [Line Items]  
Entity Address, Address Line One 1-3 Strand
Entity Address, City or Town London
Entity Address, Postal Zip Code WC2N 5EH
Entity Address, Country GB
Contact Personnel Name Justine Campbell
Country Region 011
City Area Code 44
Local Phone Number 20 7004 3000
Phone Fax Number Description 011 44 20 7004 3004