On-call and per diem work differ in one fundamental way: an on-call worker must stay available to be summoned by a single employer, while a per diem worker is hired one shift at a time and is free to accept or decline each one. That difference drives everything else — how you’re paid for time you’re not actively working, whether overtime kicks in sooner, whether you qualify for health coverage, and how much of your off-hours actually belong to you.
The Core Difference
On-call work means your employer expects you to be reachable and ready to report within a set window, even when you aren’t performing tasks. A surgical nurse might carry a pager overnight and need to be scrubbed in within 30 minutes of a call. Emergency services, utilities, and IT departments run on similar structures. The defining feature is that your employer controls some portion of your non-working time by requiring you to stay available.
Per diem means “per day” in Latin, and in employment it describes workers hired on a daily or shift-by-shift basis. A staffing coordinator posts available shifts, and you pick the ones that fit your schedule. If none appeal to you that week, you simply don’t work. Hospitals use per diem nurses to cover staffing gaps, schools hire substitute teachers this way, and construction companies bring on per diem laborers when project volume spikes. Per diem workers often maintain relationships with several employers at once, picking up shifts wherever they’re available.
Decline a per diem shift and nothing happens; you just don’t earn that day. Decline to be reachable during an on-call period and you’ve violated the terms of your job.
When On-Call Time Has to Be Paid
This is where on-call workers get confused, and where the money actually sits. The Fair Labor Standards Act doesn’t treat “on-call” as a single category. Instead, the Department of Labor asks how much freedom you actually have while you’re waiting.
On-Call at the Workplace
If you have to stay on your employer’s premises, or so close that you can’t use the time for your own purposes, that time counts as hours worked and must be paid. A hospital employee required to remain in an on-call room is working the entire time, even if allowed to sleep, read, or watch television.1eCFR. 29 CFR 785.17 – On-Call Time If waiting is part of the job and you can’t leave, you’re “engaged to wait,” and every minute counts.2eCFR. 29 CFR 785.15 – On Duty
On-Call Away From the Workplace
If you can go home and simply leave a phone number where you can be reached, that time generally does not count as hours worked.1eCFR. 29 CFR 785.17 – On-Call Time But that isn’t absolute. The DOL weighs the overall picture: how fast you must respond, whether you’re confined to a geographic area, how often you actually get called, and whether the restrictions keep you from using the time for personal activities.3U.S. Department of Labor. FLSA Hours Worked Advisor – On-Call Time An employee who must respond within 15 minutes and gets called six times per shift has a much stronger argument for compensation than one who must respond within an hour and averages one call per night.
No single factor decides it. The more your employer limits what you can do during on-call time, the more likely that time is compensable.
How On-Call Pay Is Usually Structured
On-call compensation often comes in layers:
- A standby rate, flat or reduced hourly, for hours you’re on-call but not working. Not legally required in all circumstances, but common in healthcare and utilities.
- Your full hourly rate for any time you’re actually called in and performing work.
- Call-back minimums that guarantee a set number of paid hours each time you’re called in, even if the actual task takes less. Two-hour or four-hour minimums are typical.
Some employers pay nothing unless you’re called in. That’s legal in many circumstances under federal law, which is why the compensability test above matters so much.
How Per Diem Pay Works
Per diem pay is simple. You work a shift, you get your hourly or daily rate for the hours you put in. No shift, no pay. You know exactly what you’ll earn for each day, and your weekly income is just the sum of your shifts. There is no standby pay to argue about because there is no standby obligation.
Overtime
Both arrangements fall under the FLSA’s overtime rule. If your compensable hours exceed 40 in a workweek, your employer owes at least one and a half times your regular rate for the excess hours.4Office of the Law Revision Counsel. 29 USC 207 – Maximum Hours For on-call workers, this is where the compensability question becomes real money. If your on-call hours at the workplace count as hours worked, they push you toward the 40-hour threshold much faster than if only your active hours count. For per diem workers, the calculation is cleaner: it’s the hours you actually worked on shifts you accepted.
Benefits and ACA Health Coverage
Per diem and on-call workers often sit in a gray zone for employer-sponsored benefits. The Affordable Care Act defines a full-time employee as someone averaging at least 30 hours of service per week.5Office of the Law Revision Counsel. 26 USC 4980H – Shared Responsibility for Employers Regarding Health Coverage Employers with 50 or more full-time and full-time-equivalent employees must offer affordable coverage to workers who meet that threshold or face potential penalties.
Because per diem and on-call schedules fluctuate, the IRS allows employers to use a look-back measurement method. The employer tracks a worker’s actual hours over a measurement period of 3 to 12 months, then uses that data to decide whether the worker averaged 30 or more hours per week.6Internal Revenue Service. IRS Notice 2012-58 – Shared Responsibility for Employers Regarding Health Coverage If the worker did hit the threshold, the employer must offer coverage during a subsequent stability period of at least six months, even if hours later drop. If the worker didn’t, no coverage obligation applies for that stability period.
Per diem workers who pick up enough shifts can unexpectedly cross into ACA eligibility. On-call workers face the same possibility, especially if their compensable on-call hours count toward that 30-hour average. Pay attention to your tracked hours; crossing the threshold may mean coverage you didn’t know you qualified for.
Beyond ACA requirements, per diem and on-call workers usually don’t receive the full benefits package offered to permanent full-time employees. Paid time off, retirement contributions, and employer-subsidized insurance are typically tied to regular full-time status. Specifics depend on the employer and any collective bargaining agreement.
The Other “Per Diem”: Travel Allowances
“Per diem” also refers to the daily travel allowance the federal government sets for meals, lodging, and incidental expenses. The General Services Administration publishes these rates each fiscal year, with a standard rate covering most of the continental United States and roughly 300 location-specific rates for higher-cost areas.7General Services Administration (GSA). Per Diem Rates This reimbursement system has nothing to do with per diem employment, even though the Latin phrase is the same. A “per diem position” means you work day by day. A “per diem rate” for a business trip means a daily expense allowance.
If you do receive a travel per diem, how it’s taxed depends on your employer’s reimbursement plan. Under an accountable plan, the payment is excluded from taxable wages when two conditions are met: it doesn’t exceed the federal per diem rate set by the GSA, and you submit an expense report documenting the dates, location, and business purpose of the trip.8Internal Revenue Service. Per Diem Payments Frequently Asked Questions That report must be filed within 60 days of the expense.9Internal Revenue Service. IRS Publication 463 – Travel, Gift, and Car Expenses
Any amount paid above the federal rate is treated as wages and subject to income tax, Social Security, and Medicare withholding.8Internal Revenue Service. Per Diem Payments Frequently Asked Questions The same result applies if you don’t submit the required expense report or if the employer’s arrangement doesn’t qualify as an accountable plan. In those cases, the entire per diem payment becomes taxable.
Your regular shift pay as a per diem employee is always taxable wages, regardless of what the arrangement is called. The tax rules above apply only to travel reimbursement, not to the wages you earn for working a shift.
Which One Fits You
The right fit depends on what you value more: income predictability or schedule freedom. On-call work often comes with standby pay and a stronger attachment to a single employer. You may receive some benefits, accumulate seniority, and have a clearer path to full-time employment. The cost is that your personal time isn’t fully your own during on-call periods. You might spend an entire weekend technically “off” but unable to travel or commit to plans because the phone could ring.
Per diem work maximizes autonomy. You pick your shifts, you can work for multiple employers, and your off-hours belong entirely to you. The income fluctuates, benefits are rare, and you carry more responsibility for keeping your calendar full by building relationships with multiple employers.
Whichever you’re weighing, read the written agreement before you start. A per diem contract should spell out the pay rate, cancellation policy, and any minimum availability expectation. An on-call agreement should state response time requirements, standby compensation, and how on-call hours feed into overtime calculations. The details matter more in these arrangements than in a standard full-time job, because so much is variable.