At some point in building a payments product for your SaaS platform, someone on your team will ask: "Should we use ACH or RTP?" And if you're not already deep in banking infrastructure, the answer feels like it should be obvious — faster is better, right? Use RTP.
Except cost matters. And availability matters. And some use cases where faster settlement sounds attractive actually have ACH return windows that protect you in ways instant settlement does not. The payment rail decision is genuinely nuanced, and the tradeoffs are not evenly distributed across use cases.
This is a ground-up explanation of how each major US payment rail works, what it costs, and when you'd pick it. Written for product teams, not banking professionals.
ACH: the backbone you probably underestimate
ACH — Automated Clearing House — is the network that processes the majority of US electronic money movement by volume: payroll, bill payments, direct deposits, B2B vendor payments. The network is governed by Nacha (formerly the National Automated Clearing House Association), which sets the rules for how transactions are formatted, submitted, and settled.
ACH operates on a batch processing model. Transactions submitted during a business day are collected into files and processed at defined settlement windows — typically two or three windows per day. Settlement is not immediate; it generally takes 1–2 business days from submission to funds being fully available and irrevocable. Transactions submitted after the last cutoff on a Friday won't settle until Tuesday.
Cost: Typical ACH transaction fees range from $0.20–$0.50 per transaction, with some pricing models offering lower per-transaction costs at volume. These fees are substantially lower than card networks (where interchange on a $500 transaction might run $7–12).
Return window: ACH transactions can be returned — essentially reversed — by the receiving institution. Returns can occur for up to 2 business days for standard returns (unauthorized transaction, insufficient funds, account closed), and up to 60 days for certain unauthorized consumer transactions. This return window is a risk management consideration for platforms that are disbursing funds to users: if you push money to a user via ACH and the transaction later gets returned, you've already delivered the goods or services.
Who uses it: Payroll (the dominant ACH use case), B2B invoice payments, subscription billing, direct deposit of government benefits. ACH's settlement delay is not a problem for these use cases — nobody expects their paycheck to be instant.
Same-Day ACH: the middle ground
Nacha introduced Same-Day ACH processing in phases starting in 2016, with the capability now available for both credits (pushing money out) and debits (pulling money in). Same-Day ACH allows transactions submitted before specific cutoff times to settle on the same business day rather than the next day or two.
Cutoff times matter here: Nacha defines three same-day processing windows. Submissions before 10:30 AM ET, 2:45 PM ET, and 4:45 PM ET each target settlement in the respective window that afternoon. Miss the 4:45 PM cutoff and your "same-day" transaction settles the next day.
Cost: Same-Day ACH carries a per-transaction surcharge compared to standard ACH — typically $0.50–$1.50 per transaction. There is also a Nacha-mandated interbank fee of $0.052 per same-day credit transaction that passes through to originating banks.
Transaction limit: Same-Day ACH has a per-transaction dollar cap. As of mid-2022, Nacha raised this limit to $1 million per transaction. Before that change, the cap was $100,000, which limited its use for larger B2B payments.
Who uses it: Gig economy payouts where workers want same-day earnings access, B2B invoice payments where the payer wants to avoid wire fees but needs same-day confirmation, payroll corrections where an employee missed the prior payroll cycle.
RTP: The Clearing House's real-time network
RTP (Real-Time Payments) is a network operated by The Clearing House — a banking association owned by the largest US commercial banks. RTP launched in 2017 and provides instant, irrevocable payment settlement: when a transaction is sent over RTP, the receiving bank credits the account in seconds, 24 hours a day, 365 days a year, including weekends and federal holidays.
The instant and irrevocable characteristics are fundamental to how RTP works, and they have significant design implications. Because RTP transactions cannot be reversed after settlement, there is no return window. If you push money to the wrong account via RTP, you need to request a return from the recipient — there's no bank-level reversal mechanism. This changes the risk calculus for disbursement products: you need your KYC/KYB and account validation to be correct before you send, not as a fallback after.
Cost: RTP transaction fees typically run $0.25–$0.65 per transaction for receiving banks, with fees for sending institutions varying based on volume and network agreements. From a per-transaction cost perspective, RTP can actually be competitive with ACH at lower volumes, though the pricing model varies depending on how you access the network.
Transaction limit: The standard RTP transaction limit is $1 million, though individual bank implementations may set lower limits for specific customer segments.
Network coverage: RTP has broad coverage among large US banks — institutions representing a significant majority of US deposit accounts can receive RTP payments. However, coverage among smaller community banks and credit unions is more variable. Before designing a product that depends on instant settlement for all users, it's worth confirming your user base's banking mix aligns with RTP coverage.
Who uses it: Gig worker instant payouts, insurance claim disbursements, emergency bill payments, B2B payments where the recipient needs same-minute confirmation. Any use case where the user experience depends on instant availability and the 24/7 availability is a feature, not a bonus.
FedNow: the Federal Reserve's entry
FedNow is the Federal Reserve's instant payment service, launched in July 2023. Like RTP, it enables instant, 24/7 payment settlement. The core technical capabilities are similar — immediate irrevocable settlement, 24/7 availability, up to $500,000 per transaction (lower initial limit than RTP's $1M, though configurable by participating institutions).
The primary difference between FedNow and RTP is institutional: FedNow is operated by the Federal Reserve itself, while RTP is operated by The Clearing House (a private banking consortium). From a payments infrastructure standpoint, this means FedNow is accessible to all Federal Reserve member banks directly, which theoretically extends reach to smaller institutions that may not have joined RTP. From a practical standpoint as of 2025, FedNow has made substantial progress in enrollment but is still building its participant base relative to RTP's established coverage.
Cost: FedNow pricing is set by the Federal Reserve — $0.045 per credit transfer at launch, with a $0.01 request for payment fee. These are Federal Reserve list prices; your effective cost depends on how your banking infrastructure accesses the network.
What to watch: Many financial infrastructure providers now route instant payments across both RTP and FedNow depending on which network the receiving institution participates in, using whichever provides coverage for the specific transaction. If you're using an API layer that abstracts payment rails, this routing may happen automatically without you needing to choose between the two.
Decision matrix: matching use cases to rails
| Use Case | Recommended Rail | Why |
|---|---|---|
| Recurring subscription billing | Standard ACH debit | Lowest cost; timing predictable; return window manageable with mandate |
| Payroll disbursement | Standard ACH credit | Industry standard; lowest cost; timing expectations already established |
| Gig worker / contractor payouts | RTP or FedNow | Immediate availability is a product differentiator; workers expect it |
| Loan disbursement to borrower | RTP or Same-Day ACH | Borrowers expect fast access; delay reduces product value; RTP preferred if KYB is solid |
| B2B invoice settlement | Same-Day ACH or RTP | Depends on urgency; Same-Day ACH cheaper, RTP if buyer needs immediate confirmation |
| Emergency disbursements (insurance, assistance programs) | RTP or FedNow | 24/7 availability and immediate access are core requirements |
Integration considerations: originator vs. third-party sender
How you access payment rails depends on your legal role in the transaction. Under Nacha's rules for ACH, two roles are relevant to platform companies:
Originating Depository Financial Institution (ODFI): The bank that submits transactions to the ACH network. Your platform is almost certainly not an ODFI — you need a bank partner to serve in this role.
Third-Party Sender (TPS): A company that submits ACH transactions to an ODFI on behalf of Originators (the actual payers or payees). If your platform is initiating payments on behalf of your SaaS customers, your platform may be a Third-Party Sender under Nacha rules — which carries specific registration, audit, and due diligence requirements. TPS registration is not optional; operating as an unregistered TPS is a Nacha rules violation that your ODFI is obligated to detect and address.
The TPS question often surprises SaaS teams that assumed they were simply "using" an ACH provider. The distinction matters: if you're directly submitting ACH files on behalf of your customers to a bank, you're a TPS. If you're using an API provider who submits on your behalf, the API provider may be the TPS and your platform is the originator. The legal structure of how you access the network determines your compliance obligations.
Return handling: ACH returns need to be processed and managed — whether you're re-presenting the transaction, crediting the customer, or investigating potential fraud. Return rate thresholds matter: Nacha has defined return rate thresholds (0.5% for unauthorized debit returns, 3% for administrative returns) above which an ODFI is required to take action. If your platform has high return rates, your banking partner will notice.
Payment rails are infrastructure. The right choice is usually not "the fastest one" — it's the one whose cost, timing, coverage, and risk profile fits your specific use case and user population. Getting this decision right before you build integration is much cheaper than retrofitting it after your first production incident.