How Same Day ACH works, and why the limit is the wrong thing to lead with

Ask how same day ach works and almost every answer starts with the limit. One million dollars per payment. It is the number in the headline of every explainer, including 2 I wrote, and it is the least useful fact about the system.

I had been leading with it for years. Then I read the rule that changes it and noticed that the limit had already stopped being the interesting part, because the 2 rails competing with ACH moved past it more than a year ago and nobody told the people asking me.

This is the corrected version. The limit is at the bottom where it belongs, and the thing that actually decides whether your money moves today is at the top.

Same day does not mean now

Start with this one thing. This is the misunderstanding that costs people an afternoon.

Same Day ACH does not settle continuously. It never has done, by design. It settles in windows, and the rule documentation names them: dollars settle “at several times throughout the day, 1:00 pm; 5:00 pm, and 6:00 pm Eastern Time”.

Three moments. Not a stream, not on demand, 3 fixed points in an Eastern time day. Miss the last one and same day quietly becomes next day. Nothing in your banking screen will use the word missed, and the payment will sit there looking exactly as submitted as it did an hour earlier, which is how most people find out about the windows in the first place.

That is the whole operational fact, and it is why I now ask people what time it is before I ask them how much they are sending. A payment of 40,000 dollars at 11 in the morning behaves completely differently from the same payment at 6 in the evening, and the difference has nothing to do with the amount.

Check your own bank's cut-off, not the network's. Banks impose their own deadline ahead of the network window, usually by an hour or more, and that internal cut-off is the one that binds you. I have not found a bank that publishes it prominently, which tells you something about how often it disappoints somebody.

The limit, and why it is about to stop mattering

Now the number everybody leads with. It is real, and it is about to move. I want to give it its due before I put it back where it belongs, because for a certain kind of business it is the only thing on this page that matters and no amount of scheduling advice will help them.

The Same Day ACH limit today is 1 million dollars per payment. Nacha has approved a rule raising it, and the language is plain: the limit goes “from $1 million per payment to $10 million per payment”, with an effective date of 17 September 2027.

The date carries more information than the number does, in 2 separate ways.

The first is that the industry asked for it sooner. In the request for comment, “77% of respondents either supported the proposed March 19, 2027 effective date or would prefer it to be earlier”, and the date still moved back 6 months to give participants “additional time for awareness, education, and implementation”. Support for the change itself was not marginal either: 87 per cent of respondents backed raising the limit and 81 per cent backed the 10 million figure specifically.

The second is that ACH is following, not leading. RTP “raised the per payment limit from $1 million to $10 million in June 2025”. FedNow “increased its limit from $1 million to $10 million in November 2025”. So by the time ACH arrives at 10 million in September 2027, the instant rails will have been there for over 2 years.

I had assumed the competitive picture ran the other way round. I thought of ACH as the default and the instant systems as the newcomers with restrictions. On this particular number the newcomers moved first and ACH is catching up on a 2 year delay.

How I got the emphasis wrong, and what fixed it

I had built my mental model of this system from explainers rather than from the rule, and explainers optimise for the thing that is easy to state. A single dollar limit is easy to state. A settlement schedule is not, and that is the whole problem. A settlement schedule with 3 fixed times, bank-specific cut-offs sitting an unknown distance in front of each one, and a different answer depending on whether your bank is sending or receiving, is not.

So I read the rule documentation instead, which took an afternoon and was less painful than I expected. What changed was not any single fact. It was the order: the moment I saw the settlement times written down as part of the same document that sets the limit, the limit stopped looking like the headline.

I cannot tell you whether that reordering matters for you. It depends on whether your payments are large or urgent. Those are 2 different problems with 2 different answers. A company sending 300 dollar refunds all day has a timing problem and no limit problem at all. A company sending 1 payment of 800,000 dollars a month has the opposite.

What I am confident about is that almost nobody asks which problem they have before reading about the limit, because the limit is what the page in front of them opens with.

What the higher limit is actually for

Nacha is specific about the use cases. None of them are consumer ones.

Business to business invoices up to 10 million. Tax remittances above the current ceiling. Cash concentration, where a company sweeps balances from several accounts into a central one, in either direction. Insurance claim payments above 1 million. And reversals, since a higher limit lets more corrections be made on the same day rather than the next.

Payroll funding is the one I would not have guessed. The rule notes that payroll credits are rarely above 1 million, but the file itself is usually funded by a single ACH debit that easily exceeds it, so the constraint was never on the payments to staff. It was on the 1 movement that pays for all of them.

I would not have predicted that one, and it is the detail that convinced me the rule was written by people who had watched actual payment files rather than read about them. The estimated scale is large. Nacha puts “the annual value of new ACH dollars that could be eligible for Same Day ACH” at 7 to 8 trillion.

A digression about the word instant

Some of this is just me noticing a shape rather than telling you anything useful, and the shape is that convergence tends to make the loud differences quiet and the quiet differences decisive. Nothing here helps your payment arrive, so skip it if you are in a hurry. It is the kind of thing that only becomes visible once you have all 3 systems in front of you at the same time, which almost nobody does, because almost nobody uses all 3.

We have ended up with 3 systems that all move money quickly and mean different things by it. One settles in 3 windows a day. Two settle continuously. All 3 will soon share the same 10 million ceiling, which makes the ceiling useless as a way to tell them apart, and leaves timing as the only real difference.

I keep thinking about that inversion, and I find it slightly funny in a dry way. The number everybody quotes is converging to identical across the rails, while the property that actually differs, when the money lands, is the one nobody puts in the headline. Anyway, back to the windows.

What I would do with this

Ask your bank 2 questions in writing and keep the answer. What is your internal cut-off for each Same Day ACH window, and do you offer the full network limit or a lower one of your own? Ask both in the same message, because the answers tend to come from different parts of the bank and asking twice takes twice as long. Both answers vary by bank and neither is reliably published, which means the only way to hold a provider to either of them is to have asked in writing before you needed the answer rather than afterwards, when the conversation is about what went wrong instead of about what the rules are.

Plan against the 1:00 pm window, not the 6:00 pm one. The last window is a safety net and a bad plan, because anything that needs a correction after it is submitted has nowhere left to go that day.

Split payments above the limit only if your bank agrees in advance. Structuring a single obligation into several payments to fit under a threshold is a conversation to have openly with your bank, not a trick to attempt quietly.

And do not rebuild anything for September 2027 yet. The date is 2 years out, the rule can be amended, and the limit change requires nothing from you beyond knowing it is coming.

The part I am mildly annoyed about

A short digression about disclosure, and it is not useful to your payment.

Every number in this piece comes from a published rule or a published request for comment. The 1:00 pm, 5:00 pm and 6:00 pm windows are in the documentation. The 87 per cent and the 81 per cent are in the comment results. The 17 September 2027 date is stated 3 times on the same page, once in the summary, once under details and once at the end of the technical section, which is the level of repetition an organisation uses when it genuinely does not want anybody to miss something.

The 1 number that decides whether your payment makes the window today, your own bank's internal cut-off, is not published anywhere I could find. It sits in a service agreement, or in the head of somebody on a support line. It is the only figure here you personally need. I find that hard to read as an accident across 3 different institutions, though I would not defend that as anything more than irritation, and my guess is that it is neglect rather than design.

Anyway, that is enough grumbling about disclosure, and back to what you can actually do about any of it.

What I could not establish

I could not find published data on how many Same Day ACH payments miss their intended window. That would be the number that proves or destroys my whole emphasis here, and neither the network nor the banks publish it. My suspicion is that it is meaningful, because the complaint pattern I see is about timing far more often than about limits, but a complaint pattern is not a measurement and I would not defend the leap.

I also do not know what proportion of banks offer the full 1 million limit today rather than a lower internal one. Nacha notes that originating banks “will need to determine whether to offer higher dollar limits”, which implies the current ceiling is already a bank-by-bank decision rather than a universal one, and I have not surveyed enough banks to put a figure on it.

None of this is advice on your treasury and we are not your bank. A payment that has to arrive on a specific day, against a contract with penalties attached, deserves a phone call to the bank rather than a page like this.

The thing I keep coming back to is the 3 windows. Everybody writing about this system, me included, leads with a limit that most payments never approach, and buries the 3 fixed times that decide the outcome for nearly every payment anybody actually sends. I did it for years without noticing, and I only noticed because I sat down to update a number that was about to change anyway.