While a 10% service charge is levied at most restaurants, hotels, and spas in Nepal, there's no fixed tipping rate for guided tours or treks. Nevertheless, tipping is expected (if the service merits it, of course) and it's a nice way to show your appreciation. After all, these guides and porters have ushered you and your belongings across mountain passes, expertly handled high altitude terrain, and been your only lifeline (and perhaps source of entertainment) in remote areas.
How much should you give?
It's the million rupee question. In theory, you should give what you think the guides and porters deserve, and only if they have provided you with good service. In practice, most trekkers don't know how much this is, and are afraid of either giving too little or too much.
Generally, a good tip for a guide is anywhere from about $10 to $15 USD (1000-1500 NPR) per day. For a porter, about $5 to $10 USD (500-1000 NPR) per day is enough. If service is stellar, feel free to give more. If not, give less. (Please note—these amounts apply to what your group gives communally, not individually.)
The figures above assume that you're trekking with a reputable company, which pays its staff and porters decent, living wages as well as provides insurance for them. Otherwise, the guide and porter might be relying on your tips to support themselves. If you've booked a super cheap trek, or are hiring a porter or guide for less than the standard day rate, you should tip more.
Tipping rates also depend on the length of your trek. If you're going for a week or two, follow the suggestions above; if you're planning a month-long expedition, $200-$250 USD will suffice. And if you're a solo traveler, you can lean towards the lower end of the spectrum.
Lastly, it's good to keep in mind that not all guides earn the same salary. Those who speak a specific language, who are highly experienced or who are internationally certified in wilderness survival, etc., can make up to $50-$100 USD per day. In those cases, you may want to give more: roughly 15% of their total salary for the duration of your trek.
You can either tip in Nepali Rupees (NPR) or US Dollars. Withdraw rupees from ATMs when you're in Kathmandu or Pokhara. You might find that ATMs can be finicky in Nepal, and sometimes you'll have to try a few machines before finding one that works. Alternatively, you can exchange cash or traveler's checks at a bank.
When to Give the Tip
Tips are generally given on the last night of the trek after dinner. If you're traveling with a group, you should all chip in money to give to the entire team, and then each trekker can separately give a little extra to their own porter and/or guide, especially if a special connection was forged.
When in Doubt
While these guidelines are a good starting point, don't feel compelled to stick to them. For example, if you connect more with your porter than your guide, or if your porter is the one carrying your kit bag and helping you over rocks, then feel free to switch it up. It's important to reward your guide and porter based on their good service and hard work, along with your individual experience on the trek.
If you have further questions about tipping, ask your travel specialist before you go. After all, you'll want to make sure you have enough cash — not only for tips, but also for a well-deserved beer at the end of your trek.