Honolulu County, Hawaii
USDA's soil survey flags shrink-swell clay on 12% of Honolulu County's mapped land and a shallow water table under basements on 4%, both below the national shares of 15% and 29%. None of the county is in drought on the November 3, 2026 Drought Monitor map.
FEMA's flood data list 866 flood losses here since 2016, 5% of them in buildings with basements. 57% of homes were built before 1980, against 50% nationally; the median home dates from 1977.
Data as of November 3, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, November 3, 2026
of homes built before 1980
Soil under Honolulu County
| Expansive clay (limits slab homes) | 12.4% |
| High water table (limits basements) | 3.8% |
| Septic fields very limited | 82.8% |
| Clay swell (LEP, by area) | 2.7% |
| High-swell land (LEP 6%+) | 9.7% |
| Mapped soil | 395,300 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Helemano | 6% | n/a |
| Wahiawa | 6% | n/a |
| Waikane | 3% | n/a |
| Lolekaa | 3% | n/a |
| Lualualei | 3% | n/a |
| Lahaina | 2% | n/a |
How we count · survey areas HI990 (2025-09-09)
What the numbers mean here
Rain over the 12 months to August 2026: 84.61 in, 30.54 in above the 1991-2020 average year of 54.07 in.
USDA rates 83% of the land very limited for septic absorption fields.
With shrink-swell clay on 12% and a shallow water table on 4% of the land, the soil survey points to neither problem across most of Honolulu County; conditions still vary lot by lot.
Drought
0% of the county is in drought (D1 or worse) on the November 3, 2026 map. Part of the county was in drought in 15 of the last 52 weeks; the most was 100% of its area, in 2 weeks, most recently on the December 9, 2025 map.
| Category, November 3, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 0% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
866 flood losses since 2016, $44.8 million paid. The worst year was 2026, with 273. Foundations of the buildings, where recorded: 5% basements, 57% slabs, 3% crawl spaces, 35% elevated. Median water depth outside the building: 14 in.
| Year | Flood losses |
|---|---|
| 2016 | 74 |
| 2017 | 24 |
| 2018 | 271 |
| 2019 | 17 |
| 2020 | 9 |
| 2021 | 158 |
| 2022 | 12 |
| 2023 | 11 |
| 2024 | 8 |
| 2025 | 9 |
| 2026 to date | 273 |
Rain
| Average year, 1991-2020 | 54.07 in |
| 12 months to August 2026 | 84.61 in (+30.54) |
| 2025 | 46.03 in (-8.04) |
| Wettest year since 1991 | 2004: 81.16 in |
| Driest year since 1991 | 2022: 33.30 in |
| Days with 1 inch or more, normal at WAIANAE 798 (2 mi away) | 3.6 |
Homes
| Housing units | 374,304 |
| Median year built | 1977 |
| Built before 1980 | 57% |
| Built 2000 or later | 19% |
| Owner-occupied | 60% |
| Median value, owner-occupied | $897,500 |
| Single-family detached | 45% |
What repairs cost
Published ranges from cost guides, not quotes. More sources, and where they disagree.
Nearby counties
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Kalawao County, HI | 81 mi | n/a | n/a |
| Kauai County, HI | 104 mi | 5% | 8% |
| Maui County, HI | 111 mi | 4% | 5% |
| Hawaii County, HI | 217 mi | 4% | 4% |
| Aleutians East Borough, AK | 2343 mi | 0% | 11% |
| San Francisco County, CA | 2373 mi | 0% | 3% |
| Mendocino County, CA | 2393 mi | 9% | 6% |
| Marin County, CA | 2395 mi | 20% | 6% |
Sources: USDA SSURGO (September 9, 2025) · US Drought Monitor (November 3, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status