Translation guide
A fixed charge that does not vary with usage or quantity. In Japanese, this is commonly expressed with terms like 定額 (fixed amount) or 均一料金 (uniform fee).
The most common meaning: a single price for a service or product, no matter how much you use or buy.
Literally 'fixed amount'. Used widely for flat-rate services, subscriptions, and fixed charges.
This service is unlimited for a flat fee.
I signed up for a flat-rate plan.
Means 'uniform fee'. Often used for services where everyone pays the same price, like taxis or admission.
This taxi company charges a flat fee within the city.
Literally 'fixed charge'. Emphasizes that the price is set and unchanging.
I chose a flat fee plan for my electricity bill.
A recurring fixed price for unlimited or set access, common with streaming, gyms, etc.
The 'fixed amount system'. The standard term for subscription or flat-rate services.
I use a flat-fee music streaming service.
Short for 'subscription'. Very common casual term for any flat-rate recurring service.
Specifies a monthly flat fee. Used in marketing.
Unlimited viewing for a flat monthly fee of 980 yen.
A single payment that covers everything, not recurring.
定額 emphasizes a fixed amount, often for subscriptions or ongoing services. 均一料金 emphasizes that the price is the same for everyone or every case, like a flat taxi fare. Use 定額 for most 'flat fee' contexts.
Do not translate 'flat fee' literally as 平らな料金. It sounds unnatural. Use the terms above.
動画もサブスクで見てるよ。
I watch videos on a flat-fee subscription too.