Indonesia's meteorological agency does not put the start of the wet season on a calendar in advance. BMKG, the Badan Meteorologi, Klimatologi, dan Geofisika, defines the onset of musim hujan arithmetically: one dasarian, a ten-day block, recording 50 mm of rain or more, followed by two further dasarian that also reach 50 mm. The dry season, musim kemarau, is declared by the mirror image of the same test. That is the mechanism behind every "the rainy season in Bali runs from X to Y" sentence you will read, and it is why the answer moves by weeks from year to year and by district within the island.
How BMKG declares a season
BMKG divides every month into three dasarian and uses them as the unit for declaring seasons: Dasarian I is days 1 to 10, Dasarian II days 11 to 20, and Dasarian III day 21 to the end of the month. The onset rule for the wet season, Awal Musim Hujan, is stated in the agency's own seasonal prediction book: if one dasarian records at least 50 mm of rain and the next two dasarian also record at least 50 mm, that first dasarian is set as the start; alternatively, if the first dasarian reaches 50 mm, one of the next two falls below it, but the three together total at least 150 mm, the first dasarian still counts as the start (BMKG, Prediksi Musim Hujan 2025/2026 di Indonesia, published September 2025, page 14, checked 16 September 2026).
The dry season test inverts the thresholds. Awal Musim Kemarau is the first of three consecutive dasarian below 50 mm, or a dasarian below 50 mm where one of the two that follow goes above it but the three together stay under 150 mm (same book, page 14).
Two further BMKG terms are worth carrying with you. Sifat Musim is the season's total rainfall expressed against its own 30-year average, sorted into Atas Normal above 115 percent, Normal between 85 and 115 percent, and Bawah Normal below 85 percent. Puncak Musim, the peak, is the month holding the highest three-dasarian rainfall run in a wet season, or the lowest in a dry one. "Normal" in these documents means the 1991-2020 average, which BMKG states explicitly as the reference for its season zoning.
The zone that covers Denpasar
Bali is not treated as one climatic unit by BMKG. The country is divided into 699 Zona Musim, or ZOM, each being an area identified as having relatively homogeneous seasonal behaviour, classified from 1991-2020 rainfall normals. Bali alone contains twenty of them, numbered BALI_01 to BALI_20. The zone that matters for most arrivals is BALI_19, which BMKG describes as southern Badung, southern Gianyar, southern Tabanan and the city of Denpasar — the strip containing the airport and the southern beach areas.
The table below compares the two most recent published seasons for that single zone. It sets BMKG's predicted wet season against its predicted dry season, using the parameters the agency itself publishes.
| Parameter | Wet season 2025/2026 | Dry season 2026 |
|---|---|---|
| Predicted onset | Dasarian II November 2025 (11-20 Nov) | Dasarian I April 2026 (1-10 Apr) |
| Against the normal | One dasarian later | One dasarian earlier |
| Peak month | January, same as normal | August, same as normal |
| Length | 14 dasarian, two shorter than normal | 25 dasarian, at least three longer than normal |
| Rainfall character | Normal | Bawah Normal (below normal) |
Sources: BMKG, Prediksi Musim Hujan 2025/2026, ZOM 435 BALI_19, and BMKG, Pemutakhiran Prediksi Musim Kemarau 2026 di Indonesia, published June 2026, same zone. Both checked 16 September 2026.
Read those two rows together and the shape of the year falls out. BMKG puts the normal wet-season onset for this zone at the first ten days of November and the normal dry-season onset at the second third of April, with roughly five months of wet season between them. The 2025/2026 season came a fortnight late and ran short; the 2026 dry season came early, ran long and was predicted to be drier than its own average.
The spread within Bali is the part most guides omit. Across the island's twenty zones, BMKG's predicted 2025/2026 wet-season onsets ranged from Dasarian III of September in BALI_17 (southern Bangli, southern Karangasem and northern Klungkung) to Dasarian III of November in three zones including Nusa Penida. That is roughly a two-month spread inside an island you can drive across in a morning.
What the Denpasar normals show
The Denpasar station's published climate normals describe a place where rainfall varies enormously by month and temperature barely varies at all. The figures below come from the WMO's World Weather Information Service record for Denpasar, for which BMKG is the supplying national service, and the record's stated reference period is 1961-1990 (World Weather Information Service, Denpasar climatological record, checked 16 September 2026).
The full monthly rainfall series for that station and period runs: January 345 mm, February 274 mm, March 234 mm, April 88 mm, May 93 mm, June 53 mm, July 55 mm, August 25 mm, September 47 mm, October 63 mm, November 179 mm, December 276 mm — an annual total of 1,732 mm, of which the three months December to February carry 895 mm. The table picks out the six months that decide a trip.
| Month (Denpasar, 1961-1990) | Mean rainfall | Days with rain |
|---|---|---|
| January | 345 mm | 27 |
| March | 234 mm | 20 |
| April | 88 mm | 9 |
| August | 25 mm | 4 |
| October | 63 mm | 12 |
| November | 179 mm | 16 |
Temperature is the non-variable. Across all twelve months of the same record, the mean daily maximum sits between 29.6 °C in August and 34.4 °C in April, and the mean daily minimum between 22.5 °C in August and 24.8 °C in April. There is no cold month and no cool season to plan around; the wet months are not noticeably hotter or cooler than the dry ones in this record.
One derived figure is worth doing yourself, because it changes how the word "rainy" lands. January's 345 mm falls across 27 rain days, which averages under 13 mm per rain day; December works out at roughly 12.5 mm and February at about 12.5 mm on the same arithmetic. That is a month where rain touches most days but deposits a moderate amount on each of them.
Which months are genuinely marginal
April and October are the two months where the published figures argue with the conventional wet-and-dry split. April in the Denpasar record carries 88 mm over 9 rain days, but BMKG's zone data puts the normal dry-season onset for the Denpasar zone in the second third of April, meaning the first third of the month is still climatologically wet. October carries 63 mm over 12 rain days — low total, but more rain days than May, June, July, August or September, which is the signature of a month building towards a season change rather than sitting inside one.
March and November are the two genuinely wet shoulder months that get sold as shoulder season. March at 234 mm over 20 rain days is a wet-season month by any reading of the record. November at 179 mm over 16 days is the month the wet season normally starts in the Denpasar zone, so the first third of it can look like October and the last third like December.
What the numbers do not tell you
The published climate record for Denpasar contains no information about when in the day it rains. The widely repeated claim that Bali's wet-season rain arrives as a short heavy afternoon burst is not something the monthly normals or BMKG's dasarian-based seasonal books establish either way: those documents publish ten-day and monthly totals, rain-day counts and season boundaries, and nothing about hourly distribution. If the timing of rain within the day matters to your plans, that is a question for a daily forecast, not for a climate normal.
Three further absences are worth stating plainly. The WMO record for the Denpasar station carries no humidity figure at all, so any humidity percentage you see attached to Bali months did not come from this source. The record does not state what rainfall threshold counts as a "day with rain", so the rain-day counts above should be compared with other Indonesian stations rather than with other countries' published figures. And neither BMKG's seasonal books nor the station record covers sea state, swell or surf conditions, which are handled by separate marine products and are a different question from rainfall.
There is also no BMKG prediction yet for the season most readers of this page are planning into. As of 16 September 2026, the most recent entries on BMKG's Prediksi Musim listing were the June 2026 update to the 2026 dry-season prediction and the 2025/2026 wet-season book from September 2025; no 2026/2027 wet-season book was listed (checked 16 September 2026).
Before you fix the dates
Decide first what you are actually optimising for, because the Denpasar figures point different ways. If you want the fewest wet days, the record's June to September block is unambiguous, with August at 25 mm over 4 rain days. If you want low rainfall with fewer other visitors, the argument is for the first half of April or the second half of September and the first half of October, accepting that the dry season's edges move by one or two dasarian each year.
Then check the current season, not the average. BMKG publishes its seasonal predictions per Zona Musim on the Prediksi Musim section of its climate pages, and the relevant zone for the airport, Denpasar and the southern beaches is BALI_19. Read that zone's predicted onset, peak and length, compare it with the normal column beside it, and you will know whether the season you are booking into is running early, late or ordinary. That takes about five minutes and is worth more than any month-by-month recommendation written a year in advance.