Westlake Corporation
US ˙ NYSE ˙ US9604131022

SecurityWLK / Westlake Corporation
InstitutionSei Investments Co
Latest Disclosed Ownership28,442 shares
Latest Disclosed Value $ 2,103,038
Sei Investments Co ownership in WLK / Westlake Corporation

On February 13, 2026 - Sei Investments Co filed a 13F-HR form disclosing ownership of 28,442 shares of Westlake Corporation (US:WLK) valued at $2,103,001 USD as of December 31, 2025. The entity filed a previous 13F-HR on November 13, 2025 disclosing 14,767 shares of Westlake Corporation. This represents a change in shares of 92.61% during the quarter. The current value of the position is $2,407,331 USD.

Sei Investments Co has a history of taking positions in derivatives of the underlying security (WLK) in the form of stock options.

Institutional Ownership: 13F and NPORT Filings

The Security and Class in the table below are shown exactly as filed by the investor. We do our best to track continuity of investments through acquisitions, and this will be reflected in the table as changes in names. In addition to descriptive data, performance of the investment is shown over time. To calculate quarterly performance, we first calculate cost basis of the shares purchased during the quarter, then use that to calculate gross profit. Quarterly return is Gross Profit / Starting Portfolio value.

Note that cost basis is calculated and stored in thousands, so small quarterly changes in shares frequently result in a cost basis of zero.

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Institutional Put/Call Ratios
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In addition to reporting standard equity and debt issues, institutions with more than 100MM assets under management must also disclose their put and call option holdings. Since put options generally indicate negative sentiment, and call options indicate positive sentiment, we can get a sense of the overall institutional sentiment by plotting the ratio of puts to calls. The chart to the right plots the historical put/call ratio for this instrument.

Using Put/Call Ratio as an indicator of investor sentiment overcomes one of the key deficiencies of using total institutional ownership, which is that a significant amount of assets under management are invested passively to track indices. Passively-managed funds do not typically buy options, so the put/call ratio indicator more closely tracks the sentiment of actively-managed funds.

WLK / Westlake Corporation Historical Put/Call Ratio
Disclosed Equity Positions (from 13F/NPORT Filings)
File
Date
Effective
Date
Form Security Class ID Avg Price
Paid (Est)
Reported
Shares
ΔShares ΔShares
(%)
Reported
Value
(x1000)
ΔValue
(%)
Alloc
(%)
ΔAlloc ΔAlloc (%) Cost Basis
(x1000)
Unrealized
Gain/Loss
(x1000)
2026-02-13 2025-12-31 13F WESTLAKE COM 960413102 28,442 13,675 92.61 2,103 84.80 0.0021
2025-11-13 2025-09-30 13F WESTLAKE COM 960413102 14,767 -7,979 -35.08 1,138 -34.11 0.0012
2025-08-14 2025-06-30 13F WESTLAKE COM 960413102 22,746 14,055 161.72 1,727 98.73 0.0020
2025-05-14 2025-03-31 13F/A-1 WESTLAKE COM 960413102 8,691 864 11.04 869 -3.12 0.0011
2025-05-13 2025-03-31 13F WESTLAKE COM 960413102 8,348 521 908 0.0010
2025-02-11 2024-12-31 13F WESTLAKE COM 960413102 7,827 3,851 96.86 897 50.25 0.0012
2024-11-12 2024-09-30 13F WESTLAKE COM 960413102 3,976 -10,236 -72.02 597 -70.99 0.0008
2024-08-13 2024-06-30 13F WESTLAKE COM 960413102 14,212 -559 -3.78 2,058 -8.82 0.0029
2024-05-07 2024-03-31 13F WESTLAKE COM 960413102 14,771 2,382 19.23 2,257 30.24 0.0033
2024-02-14 2023-12-31 13F WESTLAKE COM 960413102 12,389 -3,517 -22.11 1,734 -12.56 0.0028
2023-11-14 2023-09-30 13F WESTLAKE COM 960413102 15,906 -370 -2.27 1,983 1.95 0.0036
2023-08-11 2023-06-30 13F WESTLAKE COM 960413102 16,276 -209 -1.27 1,945 1.73 0.0034
2023-05-12 2023-03-31 13F WESTLAKE COM 960413102 16,485 1,334 8.80 1,912 23.05 0.0036
2023-02-10 2022-12-31 13F WESTLAKE COM 960413102 15,151 -31,608 -67.60 1,554 -61.78 0.0035
2022-11-14 2022-09-30 13F WESTLAKE COM 960413102 46,759 -20,335 -30.31 4,063 -38.21 0.0091
2022-08-15 2022-06-30 13F WESTLAKE COM 960413102 67,094 6,337 10.43 6,576 -12.11 0.0142
2022-05-13 2022-03-31 13F WESTLAKE COM 960413102 60,757 -2,139 -3.40 7,482 22.45 0.0150
2022-02-14 2021-12-31 13F WESTLAKE CHEM COM 960413102 62,896 9,471 17.73 6,110 25.26 0.0115
2021-11-12 2021-09-30 13F WESTLAKE CHEM COM 960413102 53,425 13,256 33.00 4,878 34.64 0.0102
2021-08-06 2021-06-30 13F WESTLAKE CHEM COM 960413102 40,169 21,137 111.06 3,623 114.38 0.0080
2021-05-12 2021-03-31 13F WESTLAKE CHEM COM 960413102 19,032 -1,852 -8.87 1,690 -0.82 0.0043
2021-02-08 2020-12-31 13F WESTLAKE CHEM COM 960413102 20,884 -4,116 -16.46 1,704 7.78 0.0044
2020-12-04 2020-09-30 13F/A-1 WESTLAKE CHEM COM 960413102 25,000 -4,513 -15.29 1,581 -0.13 0.0046
2020-11-06 2020-09-30 13F WESTLAKE CHEM COM 960413102 25,000 -4,513 1,581 3,806.6965
2020-08-17 2020-06-30 13F/A-1 WESTLAKE CHEM COM 960413102 29,513 29,513 1,583 0.0049
2019-08-14 2019-06-30 13F WESTLAKE CHEM COM 960413102 0 -567 -100.00 0 -100.00
2019-05-15 2019-03-31 13F WESTLAKE CHEM COM 960413102 567 142 33.41 41 28.13 0.0001
2019-02-13 2018-12-31 13F WESTLAKE CHEM COM 960413102 425 -274 -39.20 32 -37.25 0.0001
2018-11-08 2018-09-30 13F WESTLAKE CHEM COM 960413102 699 -36,647 -98.13 51 -98.73 0.0002
2018-08-03 2018-06-30 13F WESTLAKE CHEM COM 960413102 37,346 2,908 8.44 4,019 4.99 0.0137
2018-05-11 2018-03-31 13F WESTLAKE CHEM COM 960413102 34,438 -16,661 -32.61 3,828 -29.68 0.0147
2018-02-08 2017-12-31 13F WESTLAKE CHEM COM 960413102 51,099 11,697 29.69 5,444 66.33 0.0189
2017-11-06 2017-09-30 13F WESTLAKE CHEM COM 960413102 39,402 -18,215 -31.61 3,273 -14.18 0.0116
2017-07-31 2017-06-30 13F WESTLAKE CHEM COM 960413102 57,617 10,148 21.38 3,814 21.66 0.0152
2017-05-05 2017-03-31 13F WESTLAKE CHEM COM 960413102 47,469 -3,132 -6.19 3,135 10.66 0.0137
2017-02-02 2016-12-31 13F WESTLAKE CHEM COM 960413102 50,601 16,609 48.86 2,833 55.74 0.0137
2016-11-04 2016-09-30 13F WESTLAKE CHEM COM 960413102 33,992 32,781 2,706.94 1,819 3,398.08 0.0092
2016-08-11 2016-06-30 13F WESTLAKE CHEM COM 960413102 1,211 -61 -4.80 52 -11.86 0.0003
2016-05-10 2016-03-31 13F/A-1 WESTLAKE CHEM COM 960413102 1,272 881 225.32 59 180.95 0.0004
2016-02-03 2015-12-31 13F WESTLAKE CHEM COM 960413102 391 75 23.73 21 31.25 0.0001
2015-11-02 2015-09-30 13F/A-1 WESTLAKE CHEM COM 960413102 316 104 49.06 16 14.29 0.0001
2015-10-30 2015-09-30 13F WESTLAKE CHEM COM 960413102 316 17
2015-08-07 2015-06-30 13F/A-1 WESTLAKE CHEM COM 960413102 212 -27 -11.30 14 -12.50 0.0001
2015-08-04 2015-06-30 13F WESTLAKE CHEM COM 960413102 212 -27 14 0.0001
2015-05-14 2015-03-31 13F/A-1 WESTLAKE CHEM COM 960413102 239 -109 -31.32 16 -15.79 0.0001
2015-05-01 2015-03-31 13F WESTFIELD FINANCIAL INC PUT 960413102 239 239 16 0.0001
2015-02-10 2014-12-31 13F WESTLAKE CHEM COM 960413102 348 348 19 0.0002
Disclosed Put Options
File
Date
Effective
Date
Form Security Class ID Opt Avg Share Price Shares ΔShares
(%)
Value
(x1000)
ΔValue
(%)
Prior Allocation
(%)
Allocation
(%)
ΔAllocation
(%)
Cost Basis
(x1000)
Profit
(x1000)
Return
(%)
2015-05-01 2015-03-31 13F WESTFIELD FINANCIAL INC PUT 239 16 n/a n/a n/a
Legend
Shares
The total number of shares held by the institution at the end of the reporting period (the effective date). This is provided in the filing.
Value
The total value of the shares as of the effective date. This is provided in the filing.
Avg. Share Price
The weighted average share price of the shares held by the institution. We use FIFO accounting to determine this price
Class
This is the security class as indicated by the filer. There are a variety of values for this field. Common values include "EC" = equity common, "EP" - equity preferred
Allocation (%)
The percent of the institution's portfolio that this position represents. This is provided in NPORT filings. For 13F filings, we calculate it.
Change in Allocation
The change in allocation from the prior reporting period. This is calculated as current allocation - prior allocation.
Change in Allocation (%)
The percent change in allocation from the prior reporting period. This is calculated as (current allocation - prior allocation) / prior allocation.
Cost Basis
The total cost of the shares held. This is calculated as reported shares * avg. share price
Unrealized Gain/Loss
This is the market value of the position as of the effective date minus the cost basis.